To confirm & agree/add to the prior posts -
Your regular 'auto' policy will cover the trailer while it is being towed (or possibly while it is hooked-up to the tow vehicle). When it’s a stand alone vehicle and not being towed, it is likely not covered. Some may think their homeowner's policy will cover the trailer when/if it's parked in their garage. Again, likely not, or not for the full amount of the loss, depending on 'the fine print'.
When I purchased my AT Chaser I called my insurance company and asked about coverage. I purchased an 'auto' policy just for the trailer so I would be covered against loss at all times.
Related to this and insurance in general, we never really know how good our customer service/claims experience will be with our insurance company until we need what we have been paying for. After many years with AAA a couple years ago (no claims) I changed to Amica Insurance. They are big in the east but I purchased my coverage over the phone from their Northern California office. For several years in a row they have had a very high customer service rating (JD Power) - this is from people who made claims!
At the time they were only second to USAA, which only active/retired/family of military people can get. Amica now has all my policies except for my life insurance. We shall see how good they are when/if I need to make a claim ☺