Increasing Tongue Weight on Nissan NV3500 (or in general)

Did the van come with a hitch from the factory? Is the 870# also what is printed/stamped on the hitch?

Have you called Nissan to see what they say? If the hitch is rated at 870#, that might be why they put it in the manual. But, with it being in the manual there is no way to know if there is really some other reason why you couldn't put a higher capacity hitch on your van and have a higher tongue weight.