Dieselengineer,
I just went out and took off my carpeted panels behind my docking station to see what I could find out for you.
Result....I got no idea. I was wrong about the pex type pipe. Ours has two drain tubes, both are the white rv garden hose type. One is just about under the docking station and one about 6 ft in front of the driver side wheels, its the one with the valve. All I know is that in order to gravity drain my water tank it has to be open. It looks to me like Heartland has used the pex pipe everywhere there is city water pressure and only used the garden type hose for the 12v pump and drain lines.
Im going to shut up now. Maby Scott can help.
TimK