The door jamb won't tell Gross Combined Weight Rating, but will tell you Gross Vehicle Weight and Front and Rear axle weight ratings.
GCWR is in the owners manual looking at all your specific cab, drivetrain, and who knows what other things... ;)
Besides, GCWR is just mostly a 'performance' thing and you'll likely exceed your trucks payload long before you come close to the GCWR, especially towing a TT.. ;)
Good luck!
Mitch