This has been a long, LONG lasting issue with Keystone products, not just Montanas, with decals over fiberglass. Don't know of any real solution, although I know Montana/Keystone will give you replacement decals when they get really bad. Still is costly or time consuming to replace them. Look on the Montana Owner's Club forum and you'll find tons of threads & posts about this.
The only thing I've heard that is supposed to actually help is Aerospace 303 applied every three to six months, but I've never seen any factual evidence of this. It IS a really good protectant though.