You really need to go speak with YOUR insurance agent, and get it in writing.
As another gentleman pointed out, it is often different if your truck is tagged commercial or private, and it varies state to state.
If you go back and search a bit you will find where I ranted and raved about not being happy with my agent after thinking as many folks in this thread have stated, and as a matter of fact as my agent had stated, that my trailers were covered, and when I went to add a new truck, they asked if I intended to pull trailers and I would need a seperate policy for each

Needless to say I was less then happy to realize that for 4+ years I had been pulling trailers daily with the business truck, and they had been uninsured. There is also somewhat of a grey area when the trailer comes detatched from the towed vehicle.
My 4 trailers are insured on riders for the commercial policy covering both truck for about $60 a year (just liability)
They carry the full insurance of the towing vehicle when pulled behind my private vehicles.
Talk with YOUR insurance agent, in your state, and then make him show it in writing, and have him write notes, or sign yours to clarify any questions you have.
I would HATE to need it and think I had it and not have it.