I actually got the job done.
The problem was two fold. One the original hole in the frame did not allow the 90 ABS elbow to fit. So I decided to use my dremal grinder and have at it. Only needed about 1/16" anyway on both sides.
The second problem was there was not enough room to make a U back into the side of the dump assembly. Two standard 90's back to back was a half pipe wide to big. To *vent* 90's would have worked. But I ended up finding a U trap that was perfect. So it looped it right into the 1.5" grey valve.
In the end I have a modern dump system. One hose connection, one 3" black straight down, and one 1.5" grey.
Only thing I don't like is it all terminates on the curb side rear since thats where the black tank was. So hooking up to the campground sewer will be a job everytime.