Reasonably level, regardless of heat source. The issue is that the fluids inside the cooling unit circulate by gravity. If too far off level, they won't flow properly and will cause cumulative damage.
The fridge manufacturers generally say +/- 3 degrees side to side, and +/- 6 degrees front to back. But that's from the refrigerator's point of view. Since they are generally mounted sideways in an RV, that means the RV should be +/- 3 degrees front to back, and +/- 6 degrees side to side.
For a 27 foot motorhome, that means that the front must be within 17" of the rear, and one side must be within 11" of the other side. When you think about it, that's really quite far off of level. So, when parking the rig, if you can walk around comfortably in the rig without feeling like you're walking up a ramp, you're probably good enough for the refrigerator.
And those level values only apply while stationary. While moving, being out of level is not a problem, since the liquid inside the cooling unit is sloshing around and not stagnant.
Adam and Sue, and a pack of little furballs
2007 Holiday Rambler Endeavor 40PDQ Limited Edition - Cummins ISL 400
2013 Ford F-150 FX4 toad - USGear Unified Tow Brake, Roadmaster Blackhawk II Tow bar, Blue Ox baseplate
Home base near Buffalo NY, often on the road to a dog show