Battery charging and using?

We have in the driveway our used MH for a few days. 1999 Rexhall Arebus. We bought "as is" so now the fun begins. We have it hooked up to the house and the lights, air cons, microwave came on fine. We have not tested anything past that point. But...when we unplug, nothing. Generator has not started but we are at a 1/4 of a tank of gas, so hoping that is why there is nothing there when we hit the start button inside. We do own a portable one so not overly concerned if we have to plug into a portable to charge batteries up. But if we and the other owner had it hooked up to shore power, shouldn't the batteries have a charge and run at least the lights. We have read through the manual on this and it says very little. Could it be dead batteries, that may have frozen over this past winter? Could it be as simple as there is a switch somewhere we are not aware of yet? Or do the batteries, shore power, generator just switch over by themselves? Any and all thoughts would be appreciated. We will be checking all fuses, could one of those cause no charging or running off batteries? We are starting from zero knowledge, but by the time we ask 100 questions and work on fixing these things we should be professionals..lol. We are aware there are a lot of questions in there. Thanks in advance