[Carl n Susan]

A 2003 is pretty old and before many of the new features. I doubt there is a transfer switch as they are installed with a generator. And on-board generators were rare in the time. If there is a aluminum enclosure (for a generator) in the front compartment; then there is a possibility of of having a transfer switch. No enclosure; no transfer switch.

Another difference is early Montanas had a different 50 amp configuration. They had one leg of 30 amp and a second leg of 20 amps for a total of 50 amps. Today's Montanas have two legs of 50 amps each for a total of 100 amps.

The power flow is pretty straight forward. The first thing I would do is unscrew the receptacle from the rear of the Montana and check there is voltage going through the receptacle.

Assuming there is power through the rear plug, next the wiring goes to the breaker box (no transfer switch). L1 and L2 (the red and white wires)? each connect to a main circuit breaker. Ensure there is 120V at each breaker.

It sure seems like the rear receptacle is faulty.