If I understand you correctly, when you are running the generator with the AC running, the RV lights are dimming. I think your RV lights are most probably 12v not 120v. The generator shouldn't have anything to do with the 12v system except for charging your batteries. If you have 120v lights in your Rv and they are diming, then the generator is not putting up enough power to handle all the loads. do you here the generator stumble when you turn on the Ac? You can check the 120 voltage by reading the voltage on any recepticle.
Jim and Lynda, (Sophie, Jake, attack trained killer Shi-Tzus :-))
2003 Fleetwood Expedition 38N 2005 Saturn Vue