I would think the RV dealer should take it to a service center authorized to do work on the chassis. In this case that would be a Sprinter service center. I'm not up on who all services Sprinters these days, but I think it is Mercedes Benz only. Dodge dealers may still be authorized, not sure.
I would look up authorized MB Sprinter service centers on the web and see if there is one close to you. If not, I would contact Winnebago. Your dealer may have had someone do work on your new MH chassis that was not done by an authorized repair facility.
The only way I could see that happening is if the dealer is very, very far from a MB service center and the repairs were relatively inexpensive. I can't see the repairs you described as fitting that bill. Something doesn't sound right to me.
If a dealer went outside the authorized repair network and put in rebuilt parts, I would think Winnebago would want to know about that.
If this is the case, that's one sleazy dealer in a hurry to make a sale. You should call them out on this forum by name.
Please find out the answers and let us know.