Phil,
Here are the definitions of GVWR and Curb Weight. As you can see, only the curb weight is an actual weight measurement, and even it does not include water, propane, etc. - just your base vehicle weight with fuel. It may or may not be a true weight, but represents what a standard, base model of that unit should weigh. Each rig is not weighed individually. The GVWR is not an actual measurement but simply a not-to-exceed, calculated number. I'm sure Gary or Carl will correct me if I got anything wrong. Oh, and water is 8.3lbs/gal and propane is 4.2lbs/gal.
Gross Vehicle Weight Rating (GVWR)
The maximum allowable weight of the fully loaded vehicle (Base Curb Weight plus options plus cargo plus passengers). The vehicle's measured GVW must never exceed the GVWR. The GVWR along with other maximum safe vehicle weights, as well as tire, rim size and inflation pressure are shown on the vehicle's Safety Compliance Certification Label, located on the left front door lock facing or the door latch post pillar.
Base Curb Weight
Weight of the vehicle including a full tank of fuel and all standard equipment. It does not include driver, passengers, cargo or any optional equipment.