What should be immediately shut off when a white mist appears from the fixed maximum liquid level gauge while filling a mobile motor fuel or RV tank?

When a white mist appears from the fixed maximum liquid level gauge while filling a mobile motor fuel or RV tank, it indicates that the tank is nearing its maximum filling limit, and liquid propane is escaping as vapor. This situation poses a safety risk as it could lead to overfilling and potential hazards associated with LP-Gas.

Shutting off the hose and valve in this scenario is crucial because it stops the flow of propane from the source to the tank, preventing further filling and mitigating the risk of overfilling. The valve is the critical point of control for stopping the liquid transfer, while shutting off the hose ensures that no more gas can escape from the delivery system.

The other options do not address the immediate concerns of stopping the transfer of LP-Gas effectively. For example, shutting off the tank and regulator would not prevent the filling process that is already in motion, and shutting off the meter would simply halt measurement but not stop the flow, which can still be hazardous. Thus, the procedure of shutting off the hose and valve is the proper response to ensure safety during the filling process.