Yours might be different, but our '06 Phaeton dump valve has 2 buttons, one to air up the system and one to dump the system.
They're manual buttons and are for “emergency” use to air up the system in the event the electrical side malfunctions.
Take appropriate precautions to block up the coach if you're using this method so you won't be trapped underneath.
On another post, I saw the dump valve can be replaced without having to pull all the wiring. Wish I had known that because the “mechanic” that changed mine took another couple of hours to pull and reseal up the wiring.
Denny, Connie & Shadow (former barn cat made good)
2006 Tiffin Phaeton, Cat C7, Freightliner Chassis