There is no monitor or switch that cuts out if the fluid is low. What you describe doesn't sound like a fluid level problem. The pump will turn whether the fluid is high or low. Are you using the switches or the remote?? There are known issues with the control unit "locking out" manual switches. Try using the remote (ensure you have a fresh battery) to operate the slides/jacks. If the remote works and the switches don't there is a "glitch" in the control unit interlock system. That is a frequent occurrence and can be bypassed by moving one (or two) wires.
The Lippert web site has several manuals of the the hydraulic systems. You can use a drill motor to operate the pump and the slides/jacks bypassing the motor. You will have to manually open the appropriate hydraulic valves for either the slides or the jacks. The manual explains how to do this.
I highly recommend you join the Montana Owners Club Forum (www.montanaowners.com), a no charge, privately owned web site, dedicated to all things Montana. There is lots of information on the hydraulic systems (and much more) there.