[travelin texans]

The converter, which converts shore power to battery power & charges your batteries, has a cooling fan that runs when the converter is under a load.
Unless you disconnected your batteries during storage they are likely very low, once you plugged the RV into shore power the converter immediately went to working hard to charge them back up. Be sure to check the water levels (should be done at least a couple times a year) in the batteries, if when removing the caps you can see the plates add distilled water to cover them.