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.
