I have USAA homeowners for my home, auto and my cabin. The cabin is where my tractor resides and is not my primary residence. I called USAA and asked if the tractor was covered and it is covered for theft, fire, vandalism and most natural type disasters other than flooding. I was skeptical at first but they sent me a copy of my policy with the relevant stuff highlighted. As mentioned by others, it is not covered if off of my property or if involved in an accident while towing it. My auto insurance will pay a very small amount on any towed property damaged in an accident but not nearly enough to cover a tractor. I'm pretty sure it does not cover accidents incurred with tractor use, in other words, if I roll it or drive it into the pond, no coverage.
I've been trying to get an Inland Marine policy but USAA doesn't do them, even through an underwriter and Farm Bureau (etc) won't provide one if my homeowner's policy isn't with them. I'd like the extra coverage but have run into dead ends everywhere I've looked.