It is on a fuse in the 12 volt distribution panel. However, you won't find one labeled Furnace. It is a control wiring circuit. The circuit 12 volt circuit will be shared by the fridge, wh, Range hood and maybe others. T-stat off does not work? You might get lucky and find one labeled furnace as it is the largest draw because it runs the fan motor.
2011 Keystone Sprinter 323BHS. Retired Master Electrician. All Motor Homes are RV's. All RV's are not Motor Homes.