I agree with thunderusone, go to a truckstop or feedmill scale, you will be very surprised to find that you'll be WELL over 18K!!!!!
My guess is 21-23K with that rig.
My GCWR is 21,500lb. and that's with my old 3/4 ton Cummins and 38 Powerquest( I think the Cig will be a bit heavier), My 1 ton probably pushes it well over 22K. These figures are Fully Loaded with petrol and all the goodies.
Congrats on the new Cig!!!