I assume you are trying to open the generator slide with the chassis engine running?
If so, then you might have one of the large 300A fuses in the battery circuit that are blown. My old rig has two. My buddy's 2006 Affinity had more with two inverters.
My manual schematic displays their locations.
1995 38' CC Magna #5280
C8.3L 300hp Cummins, 31,000lbs
Gillig Bus Chassis