Rick,
Sorry to come to this thread late. Here are a few thoughts:
Since (I think) you have an aftermarket system, some of the "what Monaco did" stuff may have been changed during the "upgrade". If I recall correctly, your 2005 Exec has been upgraded to an Aqua-Hot 600D with dual electric elements and two of the newer 10-button thermostats, which implies that your original roof units have been changed as well. Is that correct?
In general, everything I've read about how rigs with two roof units but three Aqua-Hot zones were accomplished was that a standalone Dometic control board was embedded in a cabinet somewhere to handle the lavatory/bath zone. If my assumptions (!!) are correct, that would mean that whoever upgraded your roof units and thermostats would have had to chase down that hidden control board and upgrade it as well. That board would be connected to one of the RJ-11 data cables from either the front or rear thermostat (original would likely have been the rear). So, if that middle zone board was never found/upgraded, then you'll not see that zone on either thermostat.
Monaco would not have left the middle zone uncontrolled, and the basement circuit would circulate for that area but not likely control the power to the heat exchanger fans. But that doesn't mean the upgrade installer would have carried that forward, especially if the Aqua-Hot 600D change and the roof unit/thermostat change were done at different times. I suspect that your best next move (other than getting to the relative warmth of Q) would be to try to locate a hidden control board. I never captured the threads on this from the old site because I have three roof units, but maybe someone recalls likely locations. I would think there should be a limited number of likely places, like behind false walls in the shower or water closet wall cabinets.
George C, 2005 Exec, with very similar upgrades