Progressive took the value my local Airstream dealer....now gone...gave to them. A good service manager with a dealership knows a vintage value and should give you what you need and deserve. A good Insurance company should accept that valuation.
P.S. the dealership should not charge you for that appraisal.
"If it can't be reduced, reused, repaired, rebuilt, refurbished, refinished, resold, recycled or composted
then it should be restricted, redesigned or removed from production."