Really handy calculator here:
http://changingears.com/rv-sec-calc-trailer-weight-tt.shtmlMany factors affect towing capacity. Fill in all or as many blanks in the calculator as you can.
Items that you don't immediately think of like the rear axle ratio affect towing capacity.
For example, my GM van has a 3.73 axle ratio. That limits my GCWR to 13,000 lbs and max tongue weight to 750 lbs. If I had the 4.10 axle ratio then the GCWR would be 14,500 and max tongue weight would be 900 lbs.
Maybe some Sprinter owners have weighed their RV's when loaded and will post their numbers so you can make a guess what yours would weigh and plug it into the calculator.