Problem with slide out opening & closing

Common problem. Look for an auto reset circuit breaker something like this in the front compartment near the battery and hydraulic pump.
You may have more than one so look for the one that connects to the pump.
Available at an RV parts store and many auto parts stores such as NAPA. Easy and cheap to replace.
If you are not a member of the Montana Owners Club forum (MOC) go over there and sign up. Great place for any and all info about your Montana.
I replaced mine with a new 50 amp breaker when it started that. Lasted a year and started again. Replaced it with two 40 amp breakers wired in parallel from info on the MOC site. No more problems.

if it is all three slides, I would check your batteries.

They have an automatic reset breaker in line on the pump motor that were issues new, let alone after 9 years. You can replace that (right above the battery behind a little red rubber cover) and probably upsize it a bit.

There was actually a fix from Lippert that involved replacing the breaker with two parallel fuses, but somebody will have to jump in that remembers it better than I will need to jump in, or you can visit the Montana Owners Club.