Place to start would be the Users Guide or the Operators Manual provided by the manufacturer of the slide system. There will be a section on securing the slides while driving. Often involving ratcheting nylon straps. You might have them in your basement compartment, that were there when you bought the rig. Mine came with a fancy shmancy crank up ratcheting setup in it's own holder in one of the basement compartments.
The other way to find info is google the brand of the slides for PDF files.
What can cause them to slide out while driving is debris in the check valves. Also a leak can do it so look for oil spots around the mechanisms. Low oil generally isn't a cause of them slipping out.