Tduce, to go along with looking at the voltage, how many batteries do you have and do you have a residential fridge? If it works on battery but not on shore, that isn’t likely to be your issue. Usually the furnace will try to start 3 times, if it’s a gas issue. Has it worked on 12v before? All of the normal issues (sail switch, gas flow, low voltage) really don’t fit working on battery but not shore. Keep us posted.
Ron and Terrie Ames with Sirius - MOC #1920
2021Montana 3230CK Super Solar Legacy Package
2021 Ram 3500 Laramie Longhorn, BIM Charging
4x4, SRW, LB, Crew Cab, Pullrite 3900 Hitch