We'll need to know more - what brand and model and maybe some pictures.
I have a pretty new 3-way Dometic (don't know the model offhand). I have all the doc for that. It has an electronic panel - just some buttons on the front that switch it from electric to battery
My really old Dometic in my VW Westphalia was more complicated (pilot lights
, condensation tubes to open, etc).
So, it all kind of depends. I'll bet if you get a little more info and provide pictures - someone will be able to help.