Adding air bags: can that increase payload capacity?

Desertdog, the short completly objective answer to your specific question is no.
Regardless of whether you're a card carrying rvnet weight cop or a realist.

But they will help the rear capacity of the truck if you understand the limits and limiting factors.
By the way some react, even lying to prove their "point", you'd think the millions of sets of airbags on the market would all be recalled and the manufacturers all bankrupt and or in jail after being prosecuted by everyone from Joe RVer to the NHSTA!