GCWR is the total weight on the scale of the coach, loaded for travel WITH the towing load and any towing equipment attached. In other words...it is the weight that MAY NOT BE EXCEEDED of everything you hit the road with. If you don't fill the water tanks ...then you don't have to include their weight full... ditto propane, cargo etc. .
The only OTHER limitation on how much you can tow are:
1. The capacity of the hitch. Today's G'Towns come with a Class 3 hitch which is rated for a MAXIMUM of 5000LB pull generally. So...if you've got 6000 lbs of GVCR left after weighing your coach loaded for the road...then your tow vehicle STILL cannot be more than 5000lbs.
2. The other thing you should also know is the REAR axle weight and weight rating. (GAWR-Rear)
Tongue weight of your tow vehicle adds weight to the rear axle and you don't want to exceed this GAWR rating either. Generally most people figure 10% of tow weight as tongue weight i.e. a 3500lb car would add 350 lbs of tongue weight to the rear axle AND subtract that much from your existing CCC. A class 3 hitch generally rates for up to 500lbs of tongue weight.
Assuming your chassis is built on an F53 Ford base...the enclosed link will give you specs for all the different variations in 2007. If you know your wheelbase and GVWR all the rest of the numbers will be here: See page 3
Good luck to ya!