Once you get into older vehicles (not just RV's) insurance becomes more difficult to find. That's not just true of an assured value policy. Toss in assured value and you narrow things further. Indeed some will quote you "don't bother me go away" sorts of prices, others will simply not quote a price.
All of this is on top of how this or that company actually performs. Just as with more conventional insurance, you very much want a company that will cover a loss if you have one.
With a trailer there are various subtle distinctions that may not be obvious when casually looking at things. Does the policy cover it while in motion / hooked to a TV? What if it separates from the TV and later runs into something? (yes, I know of a claim denied this way). Does it cover the trailer in storage? How about at a camp site? How about the contents of the trailer? Flood, wind, fire, falling trees, hail, bear attack .... what is and isn't covered? Liability when the trailer does damage to something else?
Indeed there are a multitude of subtle issues past the basic "what is it worth?" question. Since there are few(er) regulations about covering a trailer, the companies sort of get to make it up as they go along ..... Also with no outside group digging into the details, there is no reference you can go to and see if policy A really is the same as policy B ....
Yes, it's a mess ....