Start with the insurer that covers your other vehicles. Most of the major insurers offer basic RV coverage, and they may offer multi-vehicle discounts.
You may get some incredulity as to why you would want to insure a 23 year old travel trailer at all, so bring along some comps for similar units to demonstrate that your Casita
does have a higher value than the typical non-molded trailer of that age.
We have all our vehicles with State Farm, and I had a conversation with my agent about 6 months ago about whether my Scamp
coverage was keeping up with inflation (especially for small molded trailers). He looked it up on his computer (probably NADA) and did a double take when he saw the number. Our 14 year old unit was worth more than the original MSRP and more that double what I paid ten years ago. It was still less than what it would take to replace in the current market, but it was enough to make the insurance worth keeping at $82/year.
If your regular insurer balks, start working your way through others. I wouldn’t expect to get Cadillac coverage on an unrestored unit that old, so you’ll bear some of the risk of loss yourself. Since insurance is not required on trailers, there’s always the option to self-insure and put some money each month in your rainy day fund instead.