Low 12V power can also cause this type of problem. If the power converter stops working and the battery is running down, you can get symptoms like this that worsen as the battery gets lower and lower.
Dan, I have my Big Country hooked to house 110V power. Could this still be the problem. Would hooking the 5th wheel to my PU maybe solve the problem?
Fred
Fred,
The power converter changes the 110V shore power to 12V to feed the 12V fuse box and charge the battery. If the converter is unplugged, or failed, or blew a fuse, or it's circuit breaker tripped, you're totally dependent on the battery to run your lights, thermostat, furnace, refrigerator circuit board, etc. even though plugged into shore power.
Before you go too far down this path, check the control panel inside the coach. In addition to showing tank water levels, it also shows lights that indicate your 12V status. If all is well, you'll see all the lights lit up.
If the control panel lights don't all light up, hooking up to the truck may help for a while. But running the furnace that way could also run down the batteries on the truck.
If the converter has failed, you can get a battery charger from Walmart and hook it up to the battery as a workaround. It may supply enough power to keep the furnace running.