Transit will be a very popular choice, but I think the rear wheel drive layout will cause some issues.
May effect room underneath for RV equip and tanks. Also big torque losses thru that configuration compared to the front drive of the RAM. So drivability may be a factor when people are test-driving finished RV's.
But they do have a longer version - so at 24 feet, you can make a more spacious RV.
The way I see it - the market will segment into 3 parts - Sprinter will be premium. Transit will be the mid-market filling in as an alternative to Sprinter on the high end. Promaster will be the economy end of the market.
But which segment will be larger? My guess is economy.