most all cooling units are built the same.
3 degrees front to back
6 degrees side to side is a number I have heard, though 6 sounds way to far off level for me. I keep it under 3.
Look at the "coil" pipes on the back of the fridge. This is where the issue lies.
There are chemicals that need to circulate through the pipes. When you drive the chemicals get sloshed around so no worry, but parked another matter.
The liquid needs to be able to flow back down this slightly sloped coil without getting stuck. if you parked on a slope and the coil now slopes up in one direction then it cannot flow. When the liquid gets stuck in the coils then the other chemical will burn or harden in the big chamber (where the flame is) and turn to a concrete-like substance. It does this slowly, Many fridges probably have some "coking" already in the chamber. Get enough and it will not cool anymore.
New cooling units are at least $600 and you still have to install.
For some model fridges there are electric conversions. Before you roll eyes, this is not like the cheap dorm style fridges you see on a used camper someone is trying to sell quick. These use a Danfoss compressor that runs off DC power (or AC) and only sip electricity. They can run off you car battery all day and still leave enough power to crank the motor. These compressors can run at 30 degrees off level. Combine with solar and a couple of batteries and you never have to refuel propane.
