It's an electric solenoid, as you know. My first step would be to see if 12v is present when the dump button is pressed. It could be a bad valve or solenoid. If no power at the solenoid, check the output wires from the dump switch. If the switch has no power, then trace the system and search for a fuse, but I'd bet the power to the leveling system supplies the dump switch.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI