Depends. If you have some type of RV that is registered as a home, it takes a proper search warrant to search it. I Think, don't quote me, the trailer may also be precluded.If he gets a SW, it must include the vehicle/trailer Written in Ink.
If you are pulling anything with an ordinary pickup; cop needs reasonable suspicion/probable cause( like, sees/smells something or a dog does), he can then search both truck, trailer, vehicle, whatever. Just because he, say, only smells pot in the tower but not trailer doesn't mean search limited to tower, only.
EDIT: Or, the owner's permission, or driver if reg. owner not present. Not a passenger's.