Does the furnace run ONLY on gas ? And same with the WH ?

With the frig, the furnace, and the WH, you need 12 vdc for the "control" system. However, the Frig can cool with either 120 vac, or propane but both need the 12 vdc. The furnace requires 12 vdc for the "control", but requires propane for heating. There is no electric heating from the furnace. Depending on the water heater, you need 12 vdc for "control", and either propane, and possibly 120 vac for heating.