Just did this.... I originally tried to measure from the remnants of the old cross beam. I've attached a pic of those measurements- when I had the new cross beam fabricated, I was going to have the guy cut the holes where they were. But I chickened out.
I ended up mocking everything up first, then marking where the holes go. I used 4.5" hole saw, based on what I could guesstimate the old holes to be. I'm not quite sure what the 4 surrounding holes were for, but I would guess they may be to access the screws on the Thetford valve body- for service, repair, replace??
I'm not going to put the surrounding holes back in. I switched to Valterra- cheap and easy. I found some 1/8 rubber squares at Lowes in the plumbing section for making your own gaskets. I will ultimately cut that to fit tightly over the plumbing that exits the rear crossmember, and also cut some aluminum to rivet, screw or bolt over it to hold it tight against the crossmember. I've attached a pic of someone's rearend
I found searching around trying to figure out what it is supposed to look like.
I dont believe the valve attaches to anything except the tank. Although, I'm with you... wondering if my entire plumbing assembly is going to be unsupported (or supported by the tank connection?? Theres a youtube video from a guy that redid his with Valterra here: Airstream valterra conversion - YouTube
I'm also going to delete the bumper plate to avoid future water intrusion. Instead I'm going to leave a gap for water to drain, but will insert an additional piece of aluminum to help shed the water away from the rear crossmember. Attached a pic of a design I found on AForums.