Medical Payments Coverage and Trailers

Our PAP insured has a travel trailer that he pulls behind his pickup truck. Recently, he fell while getting out of the trailer, which was in a campground and still attached to the truck. The insurer is denying the claim for medical payments for our insured's injury, citing the exclusion for bodily injury sustained "while 'occupying' any vehicle located for use as a residence or premises." They state that the trailer, by being located in the camp ground, has achieved residence status, and therefore there is no coverage.

May we have your opinion?