Do you have two AC Units? I would think that the second t-stat would control the second furnace. We only have one furnace but 2 AC Units. The front t-stat controls the furnace but the second t-stat only controls the rear AC unit.
Your chassis is different than mine but to clear the trip odometer I just push in on the piece pertruding through the clear plastic and hold it for a couple seconds.
In our case the water pump is over top the propane tank in a compartment that has a 4" round screw out plug. Otherwise if you can't locate it there, put water in the fresh water tank turn the pump on and open a faucet and listen for the noise of the pump.
to the Forums and congrats on the 391.
Hopefully some of the 391 owners on the forum can give you better info about the 391.
You can contact John Armstrong at FR. I've known John for years and he's always been helpful.