[arcticdude]

Isn't that a pressure protection valve? It looks like this: http://www.everythingtruckparts.com/bend...7AodnwYAHw

There should be several of those around the coach. Essentially check valves to hold pressure into a given system, while letting 2 way flow occur if the pressure is high enough on both sides of the valve (typically 65 psi). If the pressure drops low enough on either side, the vavle shuts and prevents further loss.