I am fairly certain it would work this way, at least here in PA.
I have full coverage on both the trailer and truck with Progressive.
1-Get in a wreck while towing - The truck insurance covers it.
2-Trailer gets slammed by a tree while at the campground, the trailer insurance covers it.
3-Trailer gets slammed by a tree while in my backyard, it goes to my homeowners insurance.
4-Trailer catches fire from the shoddy wiring and burns down to the frame while in my yard, its on the trailer insurance.
5-The insurance company, whomever it is, is going to attempt to put the screws to you no matter what happened, where it happened, how it happened, etc. They are going to use every play they have in the playbook.
I am also 100% certain about this. Do not get your insurance advice on an internet forum.