This is driving me absolutely insane. I have a 1980 JAYCO with a dometic furnace. My furnace blows but will not fire up on battery power. It fires on shore power, fires if I hooke my running truck up to the camper battery with jumper cables, but it just will not fire on plain only battery power. Seems to me like it just doesn't have enough oomph to trip the sail switch. -Battery is brand new 24 series deep cell from Costco. I topped off the charge when I got it home. -4 gauge wire to a kill switch and a frame ground, then new 10 gauge wire back to my power distribution center. The 'in' wire on the power center is like 14 gauge -I took the furnace completely apart and cleaned it. Made sure everything moved freely and air flowed. Sail switch moves with ease by hand. Fan spins without resistance by hand. -I cleaned all grounds and connections. I added grounds from the lift frame to the camper frame because I noticed the internal wire grounds to the lift structure. - I jumped straight to the battery from the furnace for the 12v in, and bypassed the distribution center. Still won't fire!!!!! After all of this it's the same thing, fires on shore or when my running vehicle has jumper cables to the battery, but not on just battery. What is going on here? Bad fan motor? Bad sail switch? It all works but will NOT work from the battery which is the only way i camp in this thing.