The 12 volt suggestion is a good one. Make sure his converter is .. well. converting. Make sure he has good 12.6 or better yet 13.6 volts.
next. I'd do a plug cleaning.. THere is a multi-wire connector on the circuit board. in my fridge that's in a black box lower left as you look at the back (outside) unplug and replug a few times.
Pull the "Eyebrow" (The part with buttons and lights) straight out, it just snaps in place. THat's the other end of the same cable. Unplug and replug a few times there as well.
Check for BUGS. the multi-legged biological kind.. Especially in the eyebrow.
The story goes that the reason we call computer glitches "Bugs" is that when they diagnosed the first one it was an insect caught in a relay (I've actually seen that kind of bug on a switch).
Well.. I've got that kind of "Bug" in this RV he may have as well.