It all starts with the VIN number, insurance companies break it down letter and number to see exactly what their going to cover. My guess is; it came back with a GVWR that they consider to be commercial, which lead them to question you regarding what exactly your intentions are.
Its the 'while being converted' part that insurance companies don't like. Its not your workmanship skills that's in question, nor do they care that it will be built better than an RV. They want to make sure your covered fully for the loss, so they say. While under conversion, the value of the vehicle is in question. The best you might hope for is liability coverage, then switch to full coverage once the conversion and vehicle has been re-titled/re-certified as an RV. Once its re-titled/re-certified you'll never have any problems.
If you follow though on this project, make sure you keep all your receipts, especially for the big ticket items, like appliances. This helps estimate the value and taxes paid.
If you were to purchase a $2,000 or a $200,000 vehicle built on a commercial truck frame titled as an RV there wouldn't be a problem.