"Your figures seem to indicate you'd be within the GVWR, but you may be exceeding your GCWR which would make towing the Lance unsafe."
I disagree. IMO, staying within GVWR is important, although exceeding it by a few pounds has seldom killed anybody.
GCWR however, is basically a performance number.
I used to tow a fifth wheel with a 1994 4X2 one ton dually. I decided to weigh it once, just for my own information. It was UNDER all GAWRs, truck and trailer, UNDER GVWRs, truck and trailer, but OVER the truck GCWR by 3,180 lbs!
That truck towed that rig over the Continental Divide, the Cascades, and the Sierra Nevada Range no problem.
It is interesting to note that from 1995 on, the GCWR of that truck increased every year, with minimal changes to the truck! By the year 2000, the load I was towing was well within the GCWR of the equivalent truck!
IMO, you will be fine towing your trailer. Personally, I would prefer to have a 3/4 ton diesel for that job, but that's just the way I am.
Make sure your tongue weight is adequate to prevent sway, and go have fun.