We've had all our trailers insured by Farmers with a rider attached to the truck's coverage. Costs about $150-175 more. Had two claims in the past 10+ years which were completely covered:
1. Damage to the truck & trailer #1 when high winds ripped the fiberglass tonneau off my truck (parked, unhitched, truck was facing trailer) & it caved in my hood before crashing into the trailer. About $2500 total.
2. Damage to front corner of trailer #2 when some idiot cut the turn too tight in the storage yard & clipped the nose. About $4000 total (had to replace cracked front cap). The idiot never fessed up BTW.
Not sure this answers OP's question but is real-world experience. Farmer's was fair in both cases.