Well, according to Ford, you can tow a fifth wheel to 16,300 pounds with a SRW. According to Redwood, their rigs are GVWR of 16,500.
The axles on the Redwood appear to be the same no matter what length they are, but the carrying capacity diminishes as weight of the unit gets longer. The pin weight also gets heavier by some extra.
Going up to a DRW takes you up to 24,600 it seems. They use a higher rear end ratio to haul, and to brake the load, and are just so much more capable to tow safely in all conditions.