I'm not familiar with your particular leveling/slide system. We have the HWH system that has a common electro-hydraulic pump that operates both the jacks and the slides. If your system is similar, does the pump run and the jacks still extend? That would eliminate the pump/wiring/fuses as the problem for the slides. Our pump failed once (bad motor ground) and nothing worked until it was replaced.
If the pump and jacks operate, but the slides don't, then I'd look at the solenoid(s) that control the hydraulic fluid to the slide cylinders. Are you pumping hydraulic fluid on the ground under the coach?
Just thought of another thing to check. Does your transmission have to be in Neutral with the parking brake set before the slides can be activated? If there's an interlock, you might be prevented from opening the slides.
Hope this helps.
Denny, Connie & Shadow (former barn cat made good)
2000 Itasca Horizon, Cat 3126B, Freightliner Chassis