Your issue has been talked about over the years from several different MOCers. 9 times out of 10 the cause was leaking piston seals in one of the slide cylinders. One or more seals in any of your cylinders have probably failed ... without the residual system pressure holding your slide in, it slowly works its way out a bit. There is a procedure on the Lippert website to check each slide cylinder ...
http://www.lippertrepairparts.com/pdf/0076.pdf
http://www.rvupgradestore.com/v/vspf..._test_0076.pdf