Back again. If amount of space is the prime concern, and you are sure you have enough pennies saved, your best option is the Extended Long Wheelbase Sprinter. The high top has 6 1/2 foot headroom in the empty cargo van, the super high top has another 10 inches. The dual rear wheel version of the 3500 has a GVWR of 11,030 pounds, for almost 5000 pounds of carrying capacity, and it can still tow 5000 with the V-6.
That's about 700 pounds more capacity than the equivalent Transit (DRW LWB-E High Top), and the Ford needs the diesel with optional axle ratio to exceed that towing capacity. There are no equivalent vans from Ram, Chevrolet/GMC or Nissan, as single rear wheel axles hold GVWR under 10,000 pounds at highest ratings.
Whether not GVWR matters depends on the nature of your conversion, Chevy and Nissan will still have at least 3000 pounds of cargo capacity to handle your RV build and passenger and cargo loads, and some manufacturers have successfully built "camper" (as opposed to "motorhome") conversions on 150/1500 models with 8000-8600 pound GVWR option packages.
Expect to pay about $8,000 to $10,000 more for a base level Sprinter, compared to a Chevy or Transit, and upwards of $55,000 for a heavily optioned 4x4 dually cargo van. Either is about $4000-5000 more if you want to start with a passenger van, and the extra trim and seats will add 600 to 1000 pounds to the empty weight.
