I'd take a photo for you, Robert, but there is no point. The only T is where the red Saniflush hose goes from the streetside nozzle on top of the tank to the curbside nozzle, and my camera wouldn't make an intelligible image of that 7 feet away on the other side of the coach where that T hides behind the water bay panel and the view of which from the curbside bay opening is blocked by other criss-crossing hoses. On our coach, the wet bay Saniflush fitting routes up into the cabinet under the toilet room sink via a blue hose that connects there to the vacuum breaker. A red hose comes out of the breaker and goes back down to the streetside nozzle on top of the tank.
I have no clue why your Saniflush would route back to the washer/dryer unless whoever installed the washer got the Saniflush hose behind the water bay manifold confused with the washer feed, but that still wouldn't gush onto the floor, unless they failed in the washer hookup as well. I guess an obvious question would be - does the washing machine work okay?
If your vacuum breaker is behind the washer, then it's possible the nozzles in the tank are clogged; there is a warning in the owners manual for the 2006 Monterey (page. 188) that flooding may occur, although it is ambiguous about exactly how or where. Even if the nozzle(s) failed due to plugging and the breaker allowed water back through its vent, I'd not expect full force city water flow like you describe.
My first inclination is to get after the washer/dryer installer, taking it back for a demonstration of the fault. There was already a vent and breaker system in place, and there should have been no need for new plumbing, much less another port into the black tank. Either they put in a new vacuum breaker for the Saniflush and didn't connect the input or output lines, or they added some new regulated device we don't know about, and fouled that installation. If possible in the meantime, I'd sure as heck try to get a gander behind the washer to see precisely where the water is emanating from.
Joel