When I inquired why the rates were so low, she said or truck covers the trailer when is in tow and the trailer is not much of a liability when it is sitting chocked at the campsite.
So....I was in the same situation; I've been towing a TT for 20 years and asked many times IF my TT was covered for liability and damage while hooked up to my tow vehicle. "Sure" was the answer I got for 20 years and then it happened....... I cut a corner too tight and damaged the siding on the camper with the edge of my tow vehicle.
So called up the insurance company - you guessed it..........
So now I have a separate policy to cover the TT under ALL circumstances for $150/year.
(BTW - the ins company came through and did eventually pay for the damage)
G