When we spoke to our Allstate agent about adding our '64 GlobeTrotter and the intended restoration, he suggested keeping receipts & documenting with pictures of the trailer as it progressed through it's restoration and it's value increased.
Also he suggested to revisit the value of the policy once the restoration is complete and to keep any trailer for sale ads that we come across that are similar to our own. All this in an effort to help validate the stated insured replacement value in the event of a mishap.
Not saying it wouldn't come down to a "discussion" of value later, but if you have detailed documentation of what you have then it'll be easier to negotiate.
Just my two-cents...
Shari