Shelia, call Tiffin at 256-356-0261 and ask them what to do about it. They may have experienced the problem and can tell you how to clean it. They also may suggest taking it back to the dealer and getting the pieces replaced. They can also replace it at Red Bay.
Don't delay, as you have one year from purchase to have the repairs covered under warranty. These guys are good and if the problem was theirs they will make it right. Avonite should not stain like that.