Most converters have to put down a deposit on a shell. You are right that your 'cost' would go up as you would have several hundred thousand tied up for 5+ months while the coach was in production. Not fun to sleep in some other guy's factory on plywood floors.
When the coach is done, which is tracked by Prevost on a monthly basis, the converter has to cough up the cash, often by means of a floating line of credit which is referred to in the biz as a 'floor plan.'
I haven't checked in the last couple of months, but usually there are about 70+ completed coaches on the lots looking for new homes, which strikes me as a lot of coaches in a fairly small market.