Jeff,
I always figured the 4XXX series of any brand was kindof their "ton-and-a-half" chassis, coming at in around the 14K+ lb. range on capacity. I consider the 5XXX series as "two-ton".
My E450 has around 2200 more lbs. capacity that I'm currently not using. Although this can be used for a lot of added water, I wanted too much RV chassis for the job for reasons other than weight carrying: Added transmission cooling capacity for improved transmission life, larger brake swept area for added downhill safety and longer times between brake jobs, and a lower rear differential ratio for less transmission slippage when crawling along desert roads while rockhounding. For my use, the E450 also has thicker chassis steel for added stiffness with whatever weights I do carry, which helps keep the coach platform in a plane on rutted roads.
Regarding side-by-side duals -> they have their good points and bad points. One of their good points is better flotation in soft conditions due to a smaller pounds per square inch surface loading ... which makes them "sink in" less for any given weight on the rear axle.
FWIW, I once had a heavy duty 1965 F250 "3/4 ton" Ford PU with 15 leaf springs in each rear position. I brought home (safely) uphill all the way for miles 6000 lbs., by actual scale weight, of drain rock from the quarry in the box of that truck. No diesel required either ... the long-throw 390 thumper did the job with ease.
