The power lead, before the fuse, should be connected to a point that has +12 volts when your ignition switch is in the 'Accessory' position. You'll just have to trace that wire back to that point. The wire may be disconnected or the terminal is corroded. I understand about the 'spaghetti' - the wiring in my MH looks like a snake-pit in places.
My levelers didn't work when I purchased this MH (used). Same symptoms as yours. I found that the power lead had become disconnected from the fuse panel where the installer had put it. Quite a bit of time, upside down, underneath the dash with the engine/transmission hump bending my body in directions it didn't much like.