You have either bad wiring to the selector solenoid or a bad selector solenoid.
You have one pump for the slides and leveling jacks. Thee is a bank of solenoids that select where the pump sends the hydraulic fluid. If the pump doesn't work on one piston then that cylinder is not getting the fluid sent to it. The switch inside the coach should send 12 volts to the correct solenoid. That should open the valve for the piston you want. Go to the selector valve bank which should be next to the pump. All of the piston hoses are connected to it. Trace the lines from to inoperative piston to the valve bank. Put a test light or meter on that solenoid wires and push the switch. If you have 12 volts then the solenoid/valve is bad if not you have a break in the electric feed from the switch to the solenoid.
Once you get into it it will be OK. Rocket science it is not.
Good luck and let us know what you find.