If your van is fairly young - say up to 3 years then many insurers will offer you "new for old" in which case the value will be that of the current version of your van. The insurers will have this information
If not, then either read the adverts. of use the Caravan Club facility where you can get a "Glasses" type valuation for a small fee.
It's worth keeping an eye on the value over the years, as once you are out of the "new for old" band the insured value probably need to be reduced each year. you also need to check the declaed value of any extras e.g. awning or mover as these probably need to be written down about 20% / year - i.e. down to nothing at 5 years. If you overpay, the insurers won't pay out much more than this anyway.
I'm sure there are more expert readers than I - hope this will prompt them to chip in too.
