We had a different experience. Was towing a trailer full of contents to another state after dad died; Minnesota to Arkansas. We have State Farm. Didn't get coverage on the trailer as last I knew, it was covered by the full coverage from the tow vehicle. After accident, agent said trailer wasn't covered. She tried to run contents through homeowner's policy but since we were too far from home area that didn't work.
I cannot understand the contents in a pickup not being covered...unless you ONLY have liability?
Call your insurance company and ask them if they will cover something that falls off of your trailer behind your truck, then you will understand. If it damaged someone's property, then that damage, up to the limits of your liability, would be covered. But not the item that fell off of the trailer.
People grossly misunderstand what "full coverage" is, as it really isn't "full coverage" for anything that happens.
People also under estimate the amount of liability insurance they should have, and what the ramifications are of not having adequate coverage.