Busskipper, I agree with much of what you said but not all, sorry.
An out of balance tire can dinitely only show up at certain speeds. As far as advice on this forum, well many people here have more experience with a motorhome than most truck service centers. So some good advice is available. Such as tire pressure which is seldom set by the manufacturer or dealer at the right pressure.
I do agree that the first step is going back to the dealer. Unfortunately most are not really geared to fixing chassis problems. Hopefully he can get them to send it somehwere that can address it.
You didn't mention it but class a gasser or class c both have many of the same suspension and handling issues. So I don't see that posting it here is a big deal.