You may want to check where the T-stat connects to the furnace. I'm not sure what they are labeled on an Atwood. Usually on every residential furnace I've looked at the heat terminal from the T-stat is labeled "W" there should also be an "R" terminal. Somehow you're getting 12 volts back on the W terminal. If you can send me the wiring diagram I'd be happy to look it over and see if I can help.
2007 Dodge Ram 2500 6.7L Cummins
2016 Jayco Jay Flight Elite 27BHS