Hydraulic or electric gear-driven slide? If it's a hydraulic slide, then it could be a leaking seal in the slideout cylinder or a leaking solenoid valve controlling that particular slideout. If it were a hydraulic hose leak, you should see hydraulic fluid.
You can cut a 2"x4" to length allowing for some padding (carpet, for instance) on the ends to wedge between both ends of the slideouts and the interior wall to hold the slideout in during travel. Just remember to put a tag on the slideout control switch reminding you to pull the 2"x4"s out before extending the slideout.
Rusty