On my 1995 Magna, the 274 wire starts at the fuse and runs to the HWH leveling box according to the wiring chart.
The power to the panel comes from the "Chassis" battery/ignition switch.
I assume you are getting no power reading with the voltmeter probes stuck into the two contact points of the fuse holder.
You could try to temporarily move the 274 wire to another fuse and see if that then activates the leveling system.
On my rig, I have an unused fuse socket in that panel that could be used to bypass the non-working fuse socket.
1995 CC Magna #5280
C8.3L 300hp Cummins, 31,000lbs