If you are looking for a heat only thermostat for the furnace, you can get any household one which has a battery to operate it. They have a relay inside which does the actual switching of the furnace on and off and will work well with the 12 volt
When you connect it up, wire the two furnace wires to the red and white terminals on the thermostat for proper heat operation. The Heat/cool/off switch can be set from heat to off when you want positive off when in storage.
If your thermostat operates the AC unit all bets are off and I have no advice as some of the AC units require a special thermostat and telephone wire and connector systems.