Seat belt extender is what you need and Winnebago is required to furnish it free of charge. Call them and get ugly. Federal Safety regs require them to do this for anyone who has trouble with the seat belts in your Winnie, regardless of how old it is, and yours is new. If they still say no, talk up the line to supervisors, etc., and tell them if you don't get a "solution" you will be filing a complaint through NHTSA. Some auto companies furnish these in two different lengths, so try seating positions with your wife so you can tell them about how long you need. I had a similar problem with Tiffin and when I finally got to their head of the service department I got my free extender. Had to get ugly, however.
2013 Tiffin Allegro Bus 43QGP
2011 Chevy Silverado 1500 Toad