how old is the furnace? I had a similar problem with mine when I first got it and it turned out to be bugs in the combustion chamber and the exhaust. not to say it is your issue but its worth a check. I pulled the igniter and burner, used a tiny vacuum attachment for computers to clean that side, and I blew compressed air from the outside through the setup, and did this several times until no more evidence of bugs showed up.
Bought the camper new in 2019. So 6 years old. I have taken it all apart and have not seen any evidence of bugs. Will try actually blowing it out. Thinking of replacing gas valve next.
ya, if you can't smell gas outside at the exhaust, that's a good place to start looking, does the igniter fire when you try to start it?
