Any caravan shop can service a three way fridge .
If it is not working on any form of power at all, you can try the turning it over ritual !
Inside is not gas , but ammonia based coolant, which crystallises if allowed to dry on any surface, so if the fridge hasn't been used for a while , and therefore no circulation , the crystals build up , eventually blocking the smaller pipes....so turning the fridge upside down, leaving it for a couple of hours, and then righting it for a couple of hours and doing this two or three ( or more) times, flushes the coolant through, and it washes the crystals back into the coolant, and they dissolve .
If it is working on mains , but not on gas, chances are the flu is blocked, or the jet is blocked, or partially blocked .
If it's working on gas, but not on either one, or both of the electric sources, it's an element issue , providing you are sure it is actually getting power to the element
It is also important to remember that a flame only ever burns vertically in a drought free environment, so the fridge must be within a couple of degrees of level for the flame to "hit the mark" , this however will not have much effect on electric only gas
12v feed should be fused to the relay from the main battery
http://www.ebay.co.uk/itm/2-Way-Circuit ... Sw4UtWRMVM
If it’s not working on gas , but is lighting , you need to clean the burner assy
Turn the fridge upside down, undo the burner retaining screw from the bottom of the chimney heat shield and disconnect the gas pipe , blow high pressure compressed air through the jet ( never "proggle" with anything , of you make the hole in the jet anything other than perfectly round the gas won’t squirt in the right place ) you can force WD40 or similar through it with airline
You need a big blue flame the full size of the burner mesh when you reassemble it
Mains and 12v are simply 120w ( depending on model) heating elements and are easy to change .