I think you may be confusing two fans... First, the furnace fan is 12VDC and is not controlled by a 15 amp circuit breaker (on the AC panel), but by a replaceable fuse (probably 15 amps) on the DC panel.
I'm "speculating" here, that one of two things may have occurred. First, your DW may have walked into the trailer just as the furnace was "shutting down" from a failed attempt to ignite. During the ignition cycle, the fan starts, after a "purge period", the ignitor attempts to light the flame. If the controller doesn't "sense a flame" it will make two more attempts at ignition. If, after three unsuccessful attempts, the fan will run (without heat) for another "purge period" to clear any remaining propane from the firebox. Then the furnace will remain "dormant" until the thermostat calls for heat and the sequence will recur.
The second possibility (and my guess) is that while the RV was at the dealership, the batteries were nearly "drained" and what you're hearing is the fan on the converter/charger. It's a small "computer fan" that cools the transformer in the converter. It is louder in some than in others, and is controlled by the 15 amp Circuit Breaker on the AC panel in the power distribution box. My guess is that if the fan you're hearing turns on/off with a circuit breaker (probably marked "CONV") then the fan you're hearing is the power supply/converter fan. It is normal for that fan to run when the converter output is above a "set level" (probably around 10 amps DC power. Turning on a couple of lights and having a "nearly dead battery" would be enough power demand to start the fan.
I'd suggest you check again later on to see if the fan is actually the furnace or if it's the converter. You can check by turning the thermostat to "heat" and dialing up the temperature until the furnace starts.
I'd guess by doing that, you'll hear an entirely different "fan noise".... Hope so, anyway