Furnace fan won't turn on

Yah, I don't think there is a special moisture in the gas line sensor that puts the furnace in lockout.

If you read the sequence of operation in the service manual the events that trigger lockout are

sail switch doesnt engage (indicating that the fan didn't come on)

flame sensor doesn't heat up after 3 ignition attempts (indicating ignition failure)

overtemp limit is sensed.

So moisture in the gas line would result in an ignition failure and be sensed that way. But the furnace doesn't attempt ignition until after the sail switch closes indicating that the fan is already operating.

The other condition that will keep the control board from ever trying to run the fan and start the furnace is if the battery voltage is very low, so that is one other possibility to keep in mind.

If the furnace doesn't start after 3 cycles it will lockout until reset. There are several reasons that it might not start. Lack of propane is one. Before it locks out the fan will cycle on each time it tries to start, which you would most likely notice unless you were away from the trailer.

So it wasn't a likely reason but I thought I'd suggest it anyway because once the furnace was reset it would make it appear as if you had an intermittent problem.

Offgrid, your comment about lockout got me thinking, so I found the manual and went through the trouble shooting guide. I ran out of propane the 2nd day of our trip and I'm wondering if the low gas tank had moisture in it (I was using up last year's propane) which, according to the guide, might make it go into lockout. I had a spare bottle of fresh propane, but if the old gas was still in the line at the furnace maybe it stayed in lockout until the moisture was somehow purged? Not sure what the mechanism for that would be, but it makes sense to me. (Not that making sense to me is significant.)

The furnace didn't originally go into lockout mode did it? To get out of lockout you have to turn the stat all the way off (or otherwise remove power), then back on, just turning up the temp setting won't do it.

Next time it doesn't want to run, bump, slap, kick it.  If that makes it run, you likely have a bad spot in the commutator, or open winding in the armature of the motor.