, it sounds like you had been severely running your cabin batteries down. Even though they are deep cycle, you will get longer life if they are not discharged below 50%. Specific gravity testing can tell the charge state, but while on the road, the voltage level is a good measure. While 12.75 volts is fully charged, about 12.20 volts is 50% charged. Because of this small difference, a digital meter is required right at the battery terminals. You can spend many hundreds of dollars to add the voltage and current remote option to the Magnum converter/inverter or wire the few dollar device to the terminals, which link follows. There has been many threads on this subject; do a search for them.
Digital Voltmeter | VoltMinder.com