Jon, Payload ratings are mostly a sales tool because they guess on your trucks actual weight as ordered. The new Dodges have a GVWR of 12,400. Actual payload (GVWR - actual truck weight) will depend on what you order. A plain jane regular cab 2wd automatic ('cause the autos lighter), dually will have the most available payload simply due to its lighter weight. I believe the Ford has 500lbs higher GCWR, if I remember correctly.
The "Tow rating" is the same as the "payload rating" a made up number based on a guestimate of truck empty weight. Real tow rating is GCWR minus truck weight.
With a slide in camper, you'll find that the GVWR is more important than the GCWR. You'll also find that Ford has a slight GVWR advantage and a larger GCWR advantage on the current models. I remain happy with my Dodge (especially the Cummins), but if you're going to be buying a heavy slide, you need to keep a close watch on the GCWR.
As an example, my truck weighs 8338 lbs with me (200lbs) a full tank of diesel and my usual stuff (running boards, camper tie-downs, bed liners, tools, CD's etc. etc. My truck is an '03 with a 12,000 lb GVWR, so my real payload is 3,662 lbs. My camper dry sticker weight is 2350 lbs, loaded with 50galls of water and all our stuff it is 3217 lbs.
Oh, and the Dodge is only available with one manual transmission behind the Cummins, the G56 6spd. It looks like there is a HD and non-HD choice, but the non-HD 6spd is not available for use with the diesel. I have yet to drive a G-56 truck (there new for '05), but my NV-5600 has very light clutch pressure requirements.
I would be remiss if I didn't let you know that currently the G-56's that have been ordered are on hold pending resolution of some issue (input shaft seal leak, perhaps?). G-56's on the lot are still available, but if it were me, I'd be looking for a new NV-5600 (older stock) truck if I could find one.
Cheers and happy shopping,
Bigfoot 25C9.5, 2003 Dodge 3500 Cummins, 6 spd., Wells Cargo EW1622 partially camperized.