Making a Threaded Fitting
After looking for a threaded flange for the Thetford dump valve I ended up talking with one of Andy's Technicians. He said they build them when needed. It was a relatively easy chore.
Get a flat flange (weld in type)
A 3 inch ABS threaded nipple
Some ABS cement
Cut the nipple with saw, square it up, glue it...that is it.