I was going to try this on my old 31' Dutchmen TT. It had a dump in front of the axles for the kitchen and another behind the axles for the black and shower. I just mainly wanted to eliminate the need of making connections twice when dumping. Upon removal of the under covering I couldn't find a path to route the plumbing that would allow full drainage.
I don't envy you drilling out all those rivets. I can imagine a couple being hard to access with a drill. I just know that the RV industry has make vast improvements in this century and your adaptation will be a snap. Just look at how you have rivets instead of screws and all the accurate information the salesman gave you.
Good luck but RV or not, plumbing still sucks.