Slideouts extend a few inches while being pulled

[Cindy and Ray's Grand Tour]

Our kitchen slide out (hydraulic) will extend a few inches at the bottom while we are driving down the road. It takes it about 100 miles to start to work out and gets further until I stop, enter the trailer and push the retract button. Then we are good for another 100. When it was in storage for 6 months, both the kitchen slide and the opposing dining room slide came out even further while sitting. I noticed that two other 5th wheels stored nearby had the same problem. Any idea what is causing this? Do others have the same issue? Is there an easy fix?