15% is light...go at least 20%.
And if your typical, use the GVWR. Most people are pushing much closer to the upper end of empty vs GVWR.
The extra fuel tank and other stuff in the truck is likely eating up a lot of your payload. If you want to load up the truck with a 1000lb of gear, I would be looking to move up to a 1 ton.
I'm not sure about the 2004 dodge but might be that the difference between the 3/4 ton and 1 ton SRW is just the rear springs...so while you can't officially change the ratings, upgraded springs may make if functionally identical.