Here is a picture of the spare tire cover on my Roadtrek. As you can see, it has several unique and special features. It accommodates the support tube and also has the elongated bubble bump outs molded on both sides to strengthen the fiberglass cover.
If it were mine, I would probably just buy one of those $20 or so fabric covers you see everywhere. I would buy one for a larger size tire than the 16" tire on the RT. I would then split it up from the bottom a few inches to accommodate the support tube and call it a day. An auto upholstery shop could probably make you a custom one that looked better.
Please let us know if you call Roadtrek. I would be curious what they charge for that cover. I bet $400 or so????