Check out this thread:
The following is the preventative advice given to me by azpete, a Winnieowner and HWH employee when I had the same issue with my bedroom slide:
Always fully extend your slide all the way before your retract it. Conversely, don't partially retract and then extend again. Retract all the way first before extending. And keep your switch depressed for about five seconds at each extreme.
This should help with minor creeping. It worked for a couple of years but has gotten worse and crept out about 8" in storage so I probably have a leaking retract solenoid.
Winnebagos of your and my vintage also might have defective hydraulic hoses with leaking around the metal ferrules that attach the fittings to the hoses so check your hoses for leaks. I had to replace four hoses on my 2002 35U's living room slide.