I have a 99 Itasca with 30 amp service. I suspect that yours and mine were wired similarly from the factory. I'll do my best to describe typical factory setup and what I did to mine. I suspect someone may have modified yours in a similar fashion.
Factory: Both AC's are wired into the coach. No ability to run both AC's on shore power at the same time. Selector switch (above microwave in mine) allow front or back on shore power. Both on generator.
Modified: I tapped into the circuits coming off the generator and wired the power coming out of the 20 amp breaker on the generator. This breaker is dedicated to the rear AC. Instead of it going directly to the rear AC I wired it to an outlet that is right next to the generator. The wires that go to the rear AC I added a standard 120v ac male plug on a short 18 inch piece of wire.
How to use: When not on shore power Ii plug the short pigtail into the outlet right next to the generator. The outlet supplies power when the generator is running via the 20amp breaker on the generator. When on shore power I unplug the pigtail and plug it into an extension cord and plug it into the 120/20 amp socked that is typically on the power poles. So on shore I have the main power cord from the RV plugged into the 30 amp plug on the pole and a cord running from the generator area/pig tail described earlier to the 20amp plug on the pole.
I hope this helps. Your's may be wired differently but this was a mod that went around the forums years ago and it works well.
Ryles