Hi,
What is really important is that the GVWR of the Sprinter is limiting, and the curb weight of most class B motorhomes is nearly as high as the GVWR, leaving very little extra cargo capacity for the 750 - 800 pounds of hitch weight.
So check the curb weight of your RV. Then look at the drivers door, and look up the GVWR, front and rear GAWR. If you know the front and rear axle weights (from the scale) and know the GVWR, and still have about 1,000 pounds left over for the hitch weight, keep researching.
You will also need to have the curb weight of the RV and the trailer weight be below the GCVWR of the Sprinter Chassis.
Because the Sprinters are typically front wheel drive, they have limited GCVWR. The new Transit vans will come with rear wheel drive and the same engine that Ford used in the F-150 with a 11,000 pound towing ability. I am not sure of the GCVWR of the Transit, I don't think it is nearly as high as the F-150 GCVWR, but still it can tow some weight.
In other words, I don't think that your RV will have more cargo rating than the RV itself weighs. Even if it does have 1,200 pounds of cargo rating (highly unlikely) it probably will not have a 15,000 pound GCVWR for the RV and trailer combined. The transmission is not all that strong. The Sprinters at my work only lasted about 100,000 - 110,000 miles before requiring a new $4,400 transmission. Basically they said there is nothing left in the old transmission to rebuild, replacement was the only option they could offer.
By the way, your question stated a 10 amp alternator, it is probably more like 100 - 150 amps capacity.
Good luck!
Fred.