What van is this based on. I know that many if not most of the Roadtrek are on the GM Savannah/Express van, but there may be others. I seriously question the "ease" of converting the vehicle to a dually for $1200.
One issue with duals is their habit of picking up larger rocks between the tires and causing flats. That is why many truck camper folks prefer the largest, strongest wheels and tires they can get in a single wheel installation, is for off road, even on good graded roads the chance is there, and also when pulling off into campsites. It happened to a friend of mine, they had set up the trailer in the campground and went riding and somewhere picked up a rock and he had a flat pretty quickly.
You really need to load the van up, full water and propane, top off the gas, and go to a CAT scale and pay the $12 and get some real numbers.
What specific size and type tires do you have now, and what are the max pressure and weight at that pressure as listed on the sidewall?
A dually conversion would most likely involve fender cutting and flares of some sort and possibly a different axle. It sounds like you would be using pickup truck parts.
As Mark noted this may be a solution looking for a problem.
Charles