If the fuse holds with the fixture wiring disconnected from the coach wiring; then the fixture has a short. If the fuse still blows with the fixture disconnected, the circuit has a short (Black wire grounded to frame). The white wire should be the ground and the black should be 12 volt + .
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7