should add, solar panels come in different types there's mono and poly from memory think mono are the cheap ones some have the ability to make power from bright days that are overcast while others need direct sun light, worth checking before you pay out good money
That's right, there's three types: mono-crystalline (aka single crystal), poly- or multi-crystalline, and amorphous (aka thin-film). They become less expensive per-watt in that order, but also less efficient per-area. The differences aren't as extreme as they used to be, but amorphous can still be as little as 75% as efficient as mono per area, maybe even worse with really cheapo panels; quality from brand to brand is a lot more variable with amorphous, while the quality of mono and poly are excellent almost regardless of manufacturer. On a boat or RV, size is a big consideration, so for that reason alone single crystal is preferable (you can make the same power with a physically smaller panel), but in addition to that it degrades more slowly over time. I only buy single crystal, myself, it is the gold standard and I think its strengths far outweight the extra cost, and for big staitonary systems that cost is often made back by needing smaller support structures for the same peak power.
Amorphous are the cheapest per watt when housed like the others (in a rigid frame behind glass), but they tend to lose at least 10% of their rated power in the first few months of use, then they stabilise. Some manufacturers derate fairly to account for this, some don't. But, this is the only technology of the three that can be deposited on a flexible substrate without glass, so that makes it possible to make flexible lightweight PV that rolls up. Great for stowage, but the retail cost per watt for flexible PV is generally much higher than any of the other options, partly because not many of these are made in large wattages, and because small wattages are priced much higher per watt. I wouldn't count on the flexible stuff to be as rugged as a well-made rigid panel unless you handle it pretty carefully, and amorphous is probably not going to perform as well for as many years as either of the other types.
The partial-shading aspect is really so much marketing hype in my opinion, and I would never let it be a factor in my buying decision. That "feature" is attained in mono- and polycrystalline types mostly by the use of blocking diodes built into the panel between cells, which reduces the overall panel efficiency in full sun (and whether that really matters depends on which type of charge controller you use, which is a subject for another discussion). Amorphous has a bit of this ability built-in, but then it has those other disadvantages of shorter life and much lower overall efficiency. What the marketing hype suggests to the consumer is that it would be OK to position the panels in partial shade, but in practice you only allow that to happen if it simply can't be avoided, and that is a rare instance indeed, and if that is the best you can do then expect to harvest far far less energy that way. None of them does nearly as well on an overcast day, and I'm not aware of any of the three types having any meaningful advantage in that dept. What you want to do in a mobile application is have a way to attach a wire extension to the panel so you can position it in full sun even when you want to park your rig in some shade. A roof mount is nice for cool season camping in the sun, but if you camp in the heat and want to park in shade try to make a mounting system that allows you to unmount the panel and move it into full sun as needed. If you're traveling in impoverished or high-theft areas that ability to unmount easily may mean you get to keep your PV because it's safely stowed inside your vehicle. Whether or not people know how to wire this stuff up themselves, they do know that it is very valuable and it is the kind of thing that is oh-so-easy to "liberate".
By the way, I'm almost sure someone will pipe in to talk about the new photovoltaic cell technology they heard of that is going to revolutionise the industry once they get it into production. Well, dream on, I've been in this field for 15 years and have heard that same stuff year after year. There are always methods that appear more promising in the laboratory, but I have yet to see anything revolutionary actually make it to market. What has happened are incremental improvements in production techniques, and economies of scale. When I bought my first panel, production monocrystal panels had a peak efficiency of 12-13%; nowadays the same type can have maybe 15-18% efficiency. Prices have stayed high due to competition with the rest of the semiconductor industry (those billions of computer chips) for the limited global supply of pure silicon ingot, the expensive raw material that both are made from. The construction of several new plants that increased the worldwide supply of pure silicon in just the last two years has had a far greater role in reducing prices than any of the economies of scale or improved production techniques have had over the last two decades. Why didn't they build these plants sooner if the demand was there? Well, the minimum cost to build such a plant is over 200million dollars. Silicon may be an abundant mineral, but purifying it to the degree needed for semiconductors is a very very pricey proposition.
So the lesson is, if you like the technology and want to use it, don't wait for pie in the sky someday; go ahead and buy what's available now because that new wonder tech may still be a long way off.
Any liquefied fuel like LP or NG will always have better primary energy density than any battery we could even dream of (which is why a long-range electric car is such a tough engineering challenge), so running these fridges on gas is always going to be a better choice for weight and space requirements, and with liquid fuels so cheap compared to PV, even in Europe with the much more sensible (meaning extreme) taxation on them it's still cheaper up-front by far. But electricity is the most versatile form of power, and with a PV system, once the equipment is paid for, the fuel is free. I don't opinionate on people's choices, what anyone wants to do is none of my business and they should do what their values and goals tell them to do. I'm offering some of what I know about PV and battery systems because someone might want to go all-electric even with the gas fridge they have. Over here lots of Vanagonners are opting for newer, efficient, all-electric portable fridges to replace the old Westy ones because they get so finicky while the newer stuff really is getting practical to use. That's a choice for the guy who wanted to know, I have no opinion on the matter, and I drive a plain-jane GL where we just take an ice chest when we go camping. I do know that I lived off-grid with a 1939 Servel LP fridge for ten years, but after upgrading my PV/battery system I switched to a medium-sized electric fridge because the price of LP had nearly tripled in those ten years. Not counting the cost of the additional PV, which I got a great deal on, the fridge paid for itself in avoided fuel cost in well under a year. The only thing I miss is the perfect quiet of the gas unit, but I've gotten used to the sound of the fridge cycling on and off.
As to the charge rate for lead-acid batteries: these will accept a high rate of charge, up to C/2, but I would keep the charge rate to at most C/5 to avoid overheating the plates. "C" is the battery capacity in amp-hours, so a C/5 rate for those 240 Ah Trojans would be 240/5 = 48A. Your 90A alternator should be able to spare that much in daylight (your headlights, fans, wipers, etc. will reduce the available charging amps when in use), and if so, a 50% discharged battery would theoretically reach full charge in about 2.5 hours. It won't actually reach a full, deep charge quite that fast, because the charge rate will slowly taper off as the batteries pass about 85-90% state-of-charge, so that last 10-15% takes a lot longer to put back (the batteries under charge are innately current-limiting, so they determine the declining rate during this absorption phase all by them selves if the charge voltage is held steady). But you would easily be back above 90% of full charge in that 2.5 hours of driving.
This assumes that the system is wired such that there are no wasteful voltage drops in the system, meaning the wiring servicing the battery is of sufficient size to allow the full charge current to reach the battery instead of turning a lot of into wasted heat. Always oversize you wiring and switching for that reason. And the stock Westy battery combiner relay is a sad little joke that will barely pass its rated 30A without overheating dramatically. Get a good cheap 100A-rated starter solenoid contactor to act as a combiner, or better still use one of the purpose-built combiners for yachts and RV's that combine and uncombine automatically while not wasting power. Over here the Yandina and Surepower brands are popular. Both have bidirectional combining capabilities that will also allow your PV or shore-power to charge both batteries even when it is initially applied to the cabin battery.
Well, that's enough, because that's all I know.