You're right, it's not simple. Have you weighed your fifth wheel and truck? I am surprised that you only added 200 lbs to the tongue weight with your normal load. The GC has about 2300 lbs tongue weight from the factory with standard equipment. The options will add weight to that number, and if your gross trailer weight is 15k, you would be around 3000 tongue weight (20%). If your tires are at maximum air they will support x lbs ea times 4, then subtract the weighed rear axle weight of the truck. Part of that tongue weight is carried by the front axle also, and with the Duramax engine you are at or over the front axle capacity. It's really marginal on the Chevy/GMC front axle we have. The only way to know is to weigh the whole rig, and most people I know have underestimated their weight. I ran across a chart one time that listed the carrying capacities at different inflation pressures and it seemed to drop substantially if the pressure was reduced. I just can't remember where I found it. There are axle ratings (GAWR), gross vehicle weight ratings (GVWR), and gross combination weight ratings (GCWR). Most of us are at or over at least 1 of those. The truck will handle it, and there are many pulling much heavier loads than these, but the weak link is the tires. Weigh the whole rig, as you would be driving down the road on a trip (fluids, toolbox, etc, and you and the family in it), then check your axle weights on all 4 axles. It surprised me! As I posted this I saw the post by Jeff 11-11-2004 just below this thread. At 55 lbs inflation the capacity of the tires you have on a dual application is about 1865 lbs, which is a drop of about 605 lbs per tire, or 2420 lbs less capacity on the rear axle alone. I run 55 in the rear empty, but adding 3000+ pounds to the bed would have to overload the tires.