The one way valve is internal in the hose connector , mounted on the water bay service board .
Three screws holding the connector to the board , and ( at least on my coach ) there was enough slack in the plumbing pipe to pull the connector out of the board and undo the plumbing connector .
Other installations may require you reach up behind the board to disconnect the plumbing.
EDIT: I found this exploded view of the connector , showing the one way valve in pieces.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.