Chevrolet only makes 2500, 2500 extended wheelbase, and 3500 vans, with gas or diesel engines. The 3500 van has a rear axle load capacity of 6084 lbs. which is 724 lbs. more than that of the 2500 van.
724 lbs. is trivial and most likely the result of different rims and tires and maybe a stiffer spring in the leaf pack. I gained 1000 lbs. of load capacity at the rear axle with my truck when I changed the factory 3195 rated tires out for ones rated at 3750@80 PSI.
You lose 831 lbs. of payload capacity with the diesel engine so you have less payload capacity with the 3500 with the diesel engine than with the 2500 with the gas engine.
For a B class motorhome I would be going with the Ford E-450 platform if I wanted slideouts and if not, then one using the Mercedes Sprinter platform with its 5 cylinder diesel engine.