You're probably not insured now, because you've made physical changes to the vehicle. If anything happens, I doubt they'll pay you much, if at all, except for the base unit minus depreciation.
When it comes to insurance, honesty counts. What you've done is like hiding a pre-existing condition when applying for travel medical.
You might have to call around, tell them the truth, and they might want to see the vehicle in person to better underwrite coverage, if they will cover it at all. DIY presents unusual situations when it comes to insurance?
Or, you could call around and tell them you're thinking about buying a base van and converting it, and see what response you get. That's about as far as I would go with the prevarication. Don't bother asking the ones that have already "ran".
Or, contact an Allstate agency (or other one stop insurance agency) and ask about it, and imply that you might be interested in moving all of your insurance to them, in addition to the RV coverage. Be prepared to do it, if they will cover you. I have Allstate here, and they cover the works.