Old Biscuit gave you the correct procedure.
You will probably find 12VDC on the brown wires at the solenoid during ignition. If you do, turn off the water heater LP switch and pull the brown and black wires from the two gas valve solenoids. Using an Ohm Meter, check each coil. You should read about 40 ohms. One of the coils will probably read open. If so the valve is bad.
My experience is that the inboard coil is bad 9 out of 10 times.
Usually the result of the Pressure Relief Valve dripping water on the coil for a long period of time.
2016 F-250 Crewcab 6.2L 4x4
2012 Palomino Sabre 32' Travel Trailer