That reminds me to say that my (20ish yo) Dometic is also not really picky about being level either. I've had the truck more un-level than is really comfortable inside the camper, and still had no problems with it cooling. Not sure if Dometic is doing things differently than others, but I heard all sorts of "it won't work unless it's EXACTLY level" complaints, and have found that it's just not true.
As for 12v, you really need to get good voltage to the fridge, and the factory trailer trailer wiring on any truck I've seen is NOT up to the task. Run a 10 or 12 ga wire directly from the front of the truck to the bed connector for 12v and you'll likely see a vast improvement in the way it works on 12v. If you've already done that, it's also possible that the element is dying. (Perhaps it got used a lot by a former owner?)