Thermostat for heat

If all else fails, remove the two wires from the thermostat (you may have to remove the thermostat from the wall) and twist them together. This will bypass the thermostat and tell the furnace to come on.

Mechanical thermostats have no fuse to check. Electronic thermostat have fuse and some have batteries.
If it is a small mechanical thermostat, remove cover, locate the two wires going to furnace. Connect them together. Fan should start, then in about 15-20 seconds the burner should start. Separate the two wires and the furnace should go into shut down mode and then shut completely off.
Make sure you have 12 volt DC turned on and batteries are up or main engine is running providing power to the coach system.
Make and model of rig would help.
Make and model of thermostat and furnace.

One check you can do, is remove your cover and check your fuse. My fuse looked good, I took it out and placed it back and every thing was fine.

Hi--it is a bit more involved that you stated. If the battery(ies) are not up, there may be insufficient voltage to fire it off. Is the motorhome plugged in? If not, try it plugged in. If you push up the thermostat and the heater runs, it is probably not the thermostat. Let us know what you find.