Furnace starts out great & then later not?

Where would I find the converter & how would I check it?

old biscuit gave the best answer but here is something else to check check the converter because if it is not charging the batteries the furnace will just blow cold air because even tho you are on shore power the furnace still uses the batteries to operate so my guess is either your batteries are going bad or the converter is not charging really well

Planning another winter trip deep into the snow & still need to correct this issue.

Might it be the temp switch where I set the temp?

When you startup the Furnace go listen to what the Burner/Flame sounds like when running. Steady or popping? Popping would suggest some type of airflow problem in the combustion chamber. If it's an old unit I bet the Burner is rusted away. I have an Atwood 8535 which had the same problem. Ran for awhile then would misfire and blow just cold air. The replacement Burner was $23 and was easy to install.

Possible #2: Mud dauber's nest in combustion chamber. Very common problem. Only way to see inside totally is Camera Scope.

Good Luck on the mission and Let's us know the outcome of problem.

Its always most helpful to know what make and model a furnace is :).

Could be an oil trap in the copper line to the furnace which passes gas when warm (less viscous) but not when cold (more viscous).

On this trip we are on full hook-ups running off shore power & no I have not restarted the engine or Geni.

Are you hooked up to shore power OR just running on battery power?

Running on battery power......how are you recharging battery?

Running on battery power........voltage goes low.....furnace quits.
You recharge battery in AM? furnace works cause voltage is back up

So how are you using/getting power?????

Old Biscuit has a great breakdown of the sequence involved. Here's a LINK. Access is via an external cover.

When you ran the furnace later and it worked had you driven the rig and charged up the batteries some? The sail switch is in the path of the air flow. When the battery is fully charged it has enough to operate the fan and create good air flow so the sail switch is activated to turn on the gas flow to the burner. As the batteries run down they do not put out enough air flow to cause the sail switch to activate. This well not allow the gas valve to open and let the propane flow. The sail switch has a flap over it and the air flow causes the flap to push down on the sail switch. This tells the gas valve that there is adequate air flow for the furnace to run. I have had this happen to me when the batteries start to run down.