as stated...level is an issue...venting is also an issue and with limited space, venting becomes the primary problem. When you depend on "cooling" of the refrigerant to give you the absorption affect, removal of the hot air from the upper cooling fins is imperative. I just posted photos of my install including the copper heat sync and the cooling fans on the venting thread....no problems yet.....several trips and lots of cooling cycles.
One other thing...gas refrigerators or any kind of absorption refrigerator takes quite a while to cool off itself, let alone cool anything else (at least 24 hours or more). Its a good idea to put "cold things" in the frig if you can....if not, at least have a 1/2 gal milk container frozen to aid the cooling process when your getting it started.
The gas will have optimum working at 3-5 degrees off level and the 120v function 15-18 degrees off level. This allows the hot fumes to rise unobstructed through the tubing coils.
The 12 volt function in a 3-way is primarily for traveling from one place to another and not designed to be the primary cooling function....it sucks juice so be prepared to keep the battery recharging constantly....if you don't, you'll lose the battery in a heartbeat. With a 2-way you can install an inverter in the truck and run the frig on 120v while traveling....works well but don't try and use an inverter through your cig lighter....alot of juice is running through those wires and the 12v cig wires aren't sized for a draw like the frig.....they'll fry