You’ll likely have to explore an “agreed value” policy. You will pay based on the appraised value of your unit. Every so often you can submit an updated appraisal to adjust the value, and your premium, up or down as necessary.
You must have a big dollar rig, in the hundreds of thousands for a company to want an appraisal.
Agreed/Stated Value policies are pretty simple, you state the value of insurance you want and your premium reflects that amount, no appraisal is needed, nor would it even apply at the time you insure that unit. A loss is then subject to actual cost of repair and/or a total loss is simply the stated value.
Now, let's say your custom Class A diesel pusher costs about $1.25 M, an insurer wants to ensure they aren't being taken for a ride by over insuring that rig, an appraisal could be requested, more likely than not, your agent will be taking detailed photos and sending them to Underwriting.
The time to prove any value is at the time of a loss. Most companies use independent adjusters and an appraisal may help. Another issue is who do you get that "appraisal" from, doubt you find "RV Appraiser" in the Yellow Pages, it's not a recognized "profession" and you're likely to end up with some RV dealer's opinion of a market value or cost to repair.
Best to simply show receipts for what you have, the actual cost is hard to argue against.