If there's a question about the batteries, start the generator. If the batteries are the problem, the large charging current from the generator to the batteries will at the same time put the large current into the slides.
So, if the slides work a lot better with the generator running, then the batteries are a problem. If the generator doesn't make a difference, it's probably not a battery problem. This is a lot easier than pulling the batteries and having them tested.
Bill
Nodwell RN110 out moose hunting. 4-53 Detroit, Clark 5 spd, 40" wide tracks, 10:00x20 tires, 16,000# capacity, 22,000# weight. You know the mud is getting deep when it's coming in the doors.