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 to
propane more-or-less automagically.
My really old Dometic in my VW Westphalia was more complicated (pilot
lights to
light, 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.