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.
It also may make a difference the type of insurance you are looking for. I just have basic liability, which is easy. Comp/collision may be tougher because they don't know how to place a valuation on it?
In my experience Progressive will do basic liability insurance on just about any RV.
Yesterday I meet a couple with the 2500 sprinter that got register as a RV, the reason they tell me is much cheaper insurance rates.. (Totally faded and rusty red van and they still have full coverage for about 20% more than i pay just for liability)
The insurance company and/or the DMV did not ask them for any proof (Minnesota), I need to implement my E350 since in reality is going to be a camper van for my girls more than anything..