How to Troubleshoot a Duo Therm Furnace | It Still Runs In the 1990s, Duo-Therm was one of the biggest manufacturers of furnaces for recreational vehicles. Today, many older recreational vehicles still use a Duo-Therm furnace to heat their interior. However, these heaters can sometimes malfunction. Common problems can be encountered in areas such as the furnace's Duo Therm Ac Wiring Diagram - wiringall May 19, · Duo therm wiring diagram: destiny deville: Have a duo therm ELECTRIC furnace, FULL SIZED, in a manufactured home. the unit is HEAT only, with optional a/c (we do not have a/c). the unit is in an '81 Kit Royal Oak wiringall if this helps. think the model # is cannot find ANY information on this unit, ANYWHERE. need wiring.