Yep, I weight all my trucks with me in them and 3/4 tank of fuel. Subtract that weight from you max GCWR and it will give you the real tow rating. Example: 20,000# GCWR and a weight slip stating your truck is 7,500# will leave you a real world tow rating of 12,500#
Now go back with your trailer and weigh it with the truck. Subtract this weight from your GCWR and this will tell you your max weigh you can put on the trailer without exceeding the trucks GCWR. Example: 20,000# GCWR and a weight slip stating your truck and trailer are 11,000# will leave you a legal payload of 9,000# by the trucks manufacture.
In this example if your trailer is a 14,000# GVWR and it weighs 3,500# empty as per my example it would leave you a legal payload for your trailer of 10,500#
Hope this helps.
Chris