My slides also started to creep when traveling. Main slide and then bedroom slide. Hydraulics by Dewald, which is now part of Lippert.
Sent pump to hydraulic shop, they determined that an internal check valve was leaking. No parts available! Installed additional in line valves hoping that that would work. Still creeped
I ended up drilling holes through the front and back support tubes, and inserting clevis pins to hold slide closed. To deploy slide, first I run the pump "IN" to take up slack, then remove clevis pin and extend.
'05 F250 6.0 Bullet Proofed
'02 Montana 3280RL Upgraded a bunch.