When you say E450, what exactly is it? It's not a regular van, as they stopped at E350.
As mentioned above, if its a motorhome, should be no issues with any insurance company. But, as I suspect, this is an ambulance or commercial truck body, that is why you are having trouble.
One way to get insurance companies to at least consider this, is to do a MH conversion - sleeping+cooking+bathroom facilities, get it inspected as such, and re-register it as a motorhome with non-commercial license plates.