Well,,, That certainly blows!!
But on the other hand it might sound like one valve is not right, as the pump should be building pressure but it should be directed toward the plumbing, not out the siphon?
Since it is not enough to hurt anything if we get it wrong, I would just try putting one of the valves in another position. Not knowing the history of your RV, I might suggest that the valve handles can get put on wrong and that can make it look like closed when they are really open.
I found the "mechanic" at my broker had used pliers when he found the valve would not move and that gave him enough torgue to ream out the valve handle! I know the broker and he did tell me he has trouble with getting good help!