Good ol' State Farm... Yep, I have them too. When I originally insured my trailer, they would only insure it for the value that I paid for it ($2500). Now that I have put a zillion hours into the rebuild, I asked them about upping the insurance. They told me to submit all of my receipts, and they would insure it for the grand total. Well, I went about adding up and scanning receipts, and it came to well over $20k (and I am still not finished). Have submitted the paperwork twice, but seems they have yet to up the coverage. Will have to pester them (or fire them) again one of these days.
I have read a few threads on here regarding this topic, as I recall, some others have had success getting their rebuilt trailers appraised, and the insurance company (not State Farm, as I recall) accepted that as the value.
There are some good bits of advice in this thread: