Welcome to the forum!
FYI there is an Insurance sub-forum where you might find some helpful threads:
http://www.airforums.com/forums/f351/
Also, these search results for "agreed value insurance" may lead to some productive threads/posts in other forums:
https://www.google.com/search?q=agre...=airforums.com
The first 3-4 search results are fairly recent threads.
With older Airstreams, the "book value" may not reflect the actual market value, so "agreed value insurance" is often done in advance.
Good luck,
Peter