Most RV slide outs are operated by a Hydraulic System. In some cases, especially with 5th wheels, the hydraulic system operates both the slide outs and the landing gear.
We see several cases every year where people set up their trailers on permanent lots where they level the trailer and push out the slides. The issue here is that they tend to forget about the set up until it is time to relocate their trailer.
Hydraulic systems have to be cycled on a frequent basis to keep the hydraulic seals and components lubricated in order to keep the system self lubricated. If this is not maintained, the seals can crack when the system is not used regularly for an extended period of time.
Most of these systems do not have replacement parts and will need to be replaced as a complete unit; the cost of this system replacement can run into the thousands of dollars.
To avoid an RV or motorhome slide out problem, cycle your hydraulics at least once a month by activating the slide outs or landing gear if available.