I had simular problems with intermittent operation of the thermostat, only we were just using air conditioning last summer.
VERY long story with two service tech visits, and a couple of talks with Dometic customer service, which led me to checking the connectors in the telephone type cable between the front main A/C unit and the thermostat, and that's where the problem was.
Dometic used the telephone wire and connectors because they were cheap and available, but the problem is they are not designed to really carry the amount of 12 volt
power the thermostat requires, so the connections get burned, and intermittent.
Mine was bad at the connection "box" in the telephone cable up under the inside cover of the front A/C unit. Good luck, and I hope this is of some help.