Here's a picture of one exactly like mine- it's a Dometic RM36. The controls at the bottom switch between propane/12v/110v; each mode of operation is independent of the other two. If his controls look like those it's likely the same- the RM211 is another model from the same era with the same panel.
For 110v operation, they're plugged in to an outlet usually located behind the fridge somewhere. On that note: Has he ascertained for certain that it's plugged in, and that 110v is coming in live? Sounds like there may be some confusion about proper converter setting.
