Another vote for solar.
Once you've got it, it's free. Free electricity. How good is that

And, with no moving parts or particularly delicate electronics, it's zero maintenance. And it'll be keeping the battery in tip top condition even if you leave the van idle for awhile.
I've got a semi-flexible lightweight steel-backed panel which, so far this season, I've rested against the bumper of the van and secured with pegs & padlock & chain. It kept me in power for two weeks stationary, charging laptops, phones etc.
Obviously it's better to get the panel mounted on the roof. A flexible steel panel like mine can be bonded very easily. The least messy way is to used 'Very High Bonding' double sided tape. There's flavours available for serious bonding - outdoor use, high temperatures etc. Bonding tape suppliers can advise on the right product, and it's not much more expensive than using adhesives.
The slightly trickier bit is routing the cable - if your roof is fibreglass. Fibreglass isn't very drilling-friendly, and if it gets cracked you can have a potentially spreading problem. If you're worried about that, and can find someone to do it who will guarantee their work (and your roof!), it might be worth spending the money getting that bit done professionally.
Drilling aside, here's a 'panel' that would be really easy to fit. If you have the roof space (and the cash), a pair of these would give you some decent juice. (140w out of the pair of panels, and likely 100w once it's regulated down to 12v).
http://www.maplin.co.uk/68w-flexible-so ... ing-511208" onclick="window.open(this.href);return false;
(Aswell as flexible and semi-flexible, you can also use the cheaper more common steel framed panels - but mounting those is more complicated, uglier, and may well end up costing the same or more by the time you've bought roof bars/brackets.)