From what my insurance agent told me, "if it requires registration with the DMV, it is not covered under Homeowner's and requires it's own policy." Are RV trailers plated by the DMV in your state? You need to ask your agent point blank if so and have him show you specifically the language in your policy that backs it up.
One reason (not the top or only reason) I bought a truck camper is that campers are not registered in my state and it is covered either under homeowner policy when off the truck, and automobile policy when on the truck.
Even if it is covered, you should probably increase the total payout value of your policy to include the cost of the trailer in case of a total loss.