Are you trying to run this second AC while on shore power, or from the genny? I'm wondering if you have any power to it.
If running on shore power, often there is a switch somewhere in the coach, that allows you to run the rear vs. front AC. If it's not positioned correctly, it's only going to let you power up the front AC.
If you're trying to run from the genny, often you can find a circuit breaker right on the gen set that will be tripped.
Bottom line, the rear AC on these older coaches is generally on a separate circuit. It does not power up with everything else. The coach is set up with 30a power. There were few, if any, 50a power setups in use then.
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake