The way Memic explained it to me last year.
A vehicle on a trailer is a "load", and any load on any trailer should be insured if you want to be paid for that item if it's damaged in a wreck. The tow vehicle insurance covers the trailer and load for liability only, not damage. If you want coverage against damage, both the trailer and the load must have their own policies.
I have full coverage on my trailer, it's about $50/year. It will cover damage/replacement up to $2500. It does not cover the load for full coveage, only the trailer itself.