If what you are saying is that the dial gauge off the tank itself and the monitor inside reads a level when manipulated with a magnet but they read empty when the gauge is connected to a tank with LP in it, then the visible sight gauge inside the tank is stuck or broken. The float inside the tank has a magnet that is just behind the aluminum cover where the gauge snaps onto. This requires that the tank be empty and a qualified service person remove an install a new one.
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