In my case, the truck insurance extends it's liability protection to the trailer while in tow.
The truck insurance does not cover the trailer for a loss such as fire, theft, etc.
Your policy may be different from mine.
EDIT: the post that followed mine is a good example. For instance, let's say I am backing my trailer into a camp spot, and damage the trailer by hitting a tree. In that case the truck insurance would NOT extend to cover the damage to the trailer. It would however cover the cost of replacing the tree, if the CG owner pushed it, because the liability coverage of the truck would extend to the trailer. But I'd be on my own paying for damage to the trailer, unless I have it insured on it's own policy ( which I do ).