In recent days, I have been looking at the same thing. I found this http://www.fiberglassrv.com/forums/f...mod-24914.html
The Omron relay referenced in the thread is no longer available; however, I found this alternative, DURAKOOL - DG34-1021-36-1012-F - AUTOMOTIVE RELAY, SPST-NO, 12VDC, 30A
The Coleman Mach solution will run about $200, factoring the cost of the thermostat, ceiling assembly and control kit.
Why have I been looking at this? I already have the elect-a-heat installed. I also have a Sunbeam tower heater. If I do the mod discussed on the linked forum thread to control A/C on a second thermostat, I am also thinking about adding a relay controlled outlet dedicated to the tower heater which would be controlled by the second thermostat as well. The idea is to have the ceiling unit set to low heat to distribute the heat from the tower heater. And the second thermostat would energize the ceiling unit and tower heater as needed.
The ceiling unit and the tower heater would be on separate circuits. And no propane would be used.