Phil, are you talking tire warranty or Montana warranty? For a year or two Keystone put LT tires on the Montana so I don't think there'd be a warranty problem there. As for tires, I have no idea. I know ST tires say right on the sidewall "For Trailer Use Only" but I've never seen anything to make me think there'd be a problem with tire warranty. But I don't know for sure.
When we took delivery of our 2003 3295RK it came with Load Range D tires in an odd size despite the placard saying LR E. I questioned that with Keystone until they sent me this copy of a letter they apparently had previously written. It has some info about LT and ST in it also. I thought maybe it would provide some help with your question. If anyone has trouble reading this (it's reduced to forum size requirements) email me and I'll send a larger version.