Gator, we had the same problem with our front slide last year. The bottom would slowly creep in about 1/2 inch or so. The problem was a faulty solenoid in the HWH system, letting hydraulic pressure relax and let the slide drift in. It's the first thing I would change -- the solenoids for the rear slide. They're located on the HWH hydraulic pump, and are not difficult or expensive to change, but if you haven't done it before, you might want a tech to do it.
Former Owner, 2006 36MDDS
2010 Arctic Fox 22GK Travel Trailer
2007 Toyota Tundra