Dou Therm 57915.622 won't work.
I have a 1999 Newmar American Star XL With one roof air (57915.622) and one furnace (I have not found a label on it yet) with a 4 button thermostat, that I just picked up and they didn't work at that ...
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A 23 y/o RV furnace is far from safe (I lost a BIL that way). If you're going to have him using it, make sure there's a new Carbon monoxide sensor present.
