Nice find with the camper!
If I get what your planning with the tanks it would leave the valve open all the time as to drain into the new holding tanks, like using the sewer connection at a campgrond. I think I remember reading a suggestion that one should allow the waste to fill the rv tank then dump into the sewer connection rather than letting the waste trickle through the pipe. There was a concern of stuff getting stranded in the dump pipes and drying out and clogging, letting multiple gallons through at the same time ensured a clean flush.