Assume what you are saying is the thermostat has stopped working. Two quirks about the Dometic heat pump--1]only one zone can/will control the furnance [usually defaulted to front or zone 1; and, 2] the back unit is slaved to the front unit via the "dip" switch settings on each roof unit.
Guess I would also be curious as to why you "suspect" the thermostat is the problem over the furnance, eg, fuse, fan motor, propane, etc.
2003 40' MDTS
Garden Ridge, Texas