Functionally they work the same on gas or elec. The only difference is the energy source. Point being, needs to be fairly level either way.
Dave, Dometic is still in business, one of the more popular brands. A full set of operating directions likely available if you Google the refer model number.
Regarding leveling, you want perfect, but half a bubble off is OK. Use the level placed on the shelf in the freezer. If it doesn't have one, use the floor of the freezer. Once you know where it's at using that for reference, you can check the floor or counter tops to see if they match what you have in the freezer. If so, you can use those to level in the future (easier/quicker). -Al
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake