Is it just the 12-volt lighting that won't work from the battery's, or all of the 12-volt accessories? Water pump, roof vent fans, etc.
After making sure the batteries are good and hold a charge, check for corroded connections and terminals. If you have a multi-meter or basic 12-volt test light, start probing around from the battery compartment into the coach to see if you can find where you stop getting power. Not sure about others, but my 72' has a circuit breaker between the house battery to the rest of the coach located in the battery compartment. That could cause an open circuit. Also if your using a lead-acid battery you should check the water level in the battery. Pop the caps off the top of the battery and the water level should be above the lead plates. If the water level is very low and it has been for a good amount of time, the battery is probably toast. Let us know what you find.