Med Pay under the Garage Coverage Form

Our insured is an auto dealer. They were using a dealership owned vehicle to haul a nonowned trailer with a float attached in a local parade. The insured driver strayed from the parade route and struck an overhead tree limb resulting in the float passengers being thrown overboard. Our question is whether or not the auto medical payments coverage applies. The policy has symbol 22 for medical payments. Although the vehicle is owned, the trailer is not.

Symbol 22 describes a covered auto as “only those autos you own (and for liability coverage any trailers you don't own while attached to power units you own). The medical payments coverage form indicates that we have medical payments for covered autos (not defined on the medical payments endorsement). We are confused. My inclination is to provide coverage for medical payments since the vehicle (trailer) is a covered auto for liability, but I am not sure that is correct. What is your opinion?