This is a constant struggle...here is my approach.
I keep an extensive file of all maintenance records and costs, I have an appraisal that I update periodically, and I stand ready to invoke the "appraisal clause" of my policy if and when I need to file a big claim. You have the right to a third party appraisal of the settlement when the Ins. Co. adjuster tries to total or low ball your settlement. You just have to tell them at the time of the attempted settlement that you want to do that. There are third party insurance claim appraisers (small fee) that will help get to a realistic settlement based on the real market value of the rig. If you have A LOT of money in the rig, you will want the Hagerty style coverage subject to their use terms. I have NOT found an auto insurer that will provide what most of us would like which is Stated Value coverage without silly use restrictions.
If there is one, please let me know!!