Problem solved, so I'm posting my own solution for reference just in case someone else has a similar problem. When the RV is manufactured and the AC's are installed there are control boards in the AC's that send configuration signals to the Duo-Therm. These configuration signals, at least in part, are set by DIP switches. The DIP settings include (but are not limited to) zones (in the case where there is more than one AC Unit) and furnace (can be assigned to a particular zone). Interestingly enough, the design of the Dometic AC unit installed on this RV, and the physical orientation of the control board makes the "ON" setting of the DIP switch in the up position. Gravity, road bumps, jostling of any kind can, ever so slightly, cause the DIP switch to drop from the "ON" setting to somewhere between "ON" and "OFF". In my case, I was parked but walking through the RV (which is very little in terms of jostling). It was nearly imperceptible to the naked eye when I looked at the DIPs. However, by cycling the DIP from "ON" to "OFF" to back "ON" it reseated itself. I suspect this will happen again (and probably when I least want it too since I use my RV year round). A less than qualified mechanic might have diagnosed the control boards as faulty (now beyond end of life and not manufactured) and would have replaced them at a cost of more than $1000 (parts and labor estimate I was given). Even if the DIP was bad, by having two AC units and a single furnace, I had the option of switching the two AC control boards with one another...only the Zone 1 had a furnace so only Zone 1 control board needed to have the furnace DIP set to "ON". If I have to reset again, total time to perform this reset is >30min and mechanical/electrical difficulty is low. In other words, it is easy to do, you just need to know this can happen. Hope this helps anyone experiencing a similar problem.