David, mine is configured similarly to Richards other than the passenger side air valve dumps the waste tank only unless you have the driver side outlet valve closed and the driver side waste valve open. If you study the plumbing, you will likely find that everything except the toilet goes into the pipe that connects between the driver side outlet valve and the valve next to the waste tank.
I have seen Newells where there was a air dump valve on both sides.
My valves are Thetford valves rather than Dupree but they may have been replaced at some point since the outlet doesn't line up quite right. Since you mentioned locking the valve (although the Thetford valves do have tabs to lock the handle down.
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