Hey Andy, first off I'm glad you posted that picture of the back of that Showhauler. I had looked pretty strongly at buying that exact rig a few years ago. I even made him an offer but the owner (it was on consignment to busesandmore) wouldn't budge. Good to see they finally sold it. I recently saw it on racingjunk for $20k less than what I had offered him. I guess finally pricing it to sell worked for them.
Anyhow, my rig has a very similar hitch. It only came with one attachment that had a 2 5/16" ball welded on.
I wanted to be able to tow things with a 2" ball, and I also often use a bike rack that goes into a standard 2" square receiver and wanted to be able to use that on my camper. So, I have a neighbor that's a retired iron worker and welder. He made me up this:
He used 1/2" plate steel. Cut it with his oxy/acetylene torch. Welded it up. I painted it. There ya have it. Works great. He wouldn't let me pay him for it but I imagine you should be able to find a welder that could make one up for you for pretty cheap. Just print out this picture maybe and have him take a look.