My current auto company (Progressive) does not cover travel trailers as a separate rider or policy. When in tow, they will cover a max of $500 for the trailer. I'm covered legally, as in I can get a license plate, but that coverage is obviously woefully inadequate. Nothing for personal property (the contents of the trailer), no coverage when the trailer is not in tow, pittance while it is in tow.
I have found only one company that will cover the trailer as a separate policy, which gets me good coverage while not towing, but in order to have coverage while towing, I would have to move my tow vehicle coverage to them as well, and they are substantially more expensive than my current company.