I had NRC in Middlebury convert my low mileage Freightliner (34000 miles)and there was no way I could get a truck like this for your kind of dollars. As far as the conversion costs at a dealer level it is going to be significantly more than your estimate.
Take a look on a site called RacingJunk, there are usually some good deals on what you are looking for. PS it is referred to as a "garage unit" as opposed to a motorhome.
Happy hunting and welcome to the forum.