Ok... yes, that GCWR looks low for your truck. Should be more than that.
And not all sure about the calculator, but if you still have your 2 CAT scale tickets (for tow vehicle, and for tow vehicle + RV)... here's how you can figure it all out. Just substitute your numbers for mine.
Tow ratings for my 2014 Ford F-450, Crew Cab, Long Box, 4x4, DRW, with a 4:30 rear end:
2014 Ford Super Duty | View Towing Specifications | Ford.com
GCWR: 33,000 lbs
5th Wheel Tow Weight Rating: 24,700 lbs.
Ductchmen Voltage 3200 GVWR: 16,500 lbs.
Now here are my 2 CAT tickets... first for just my truck, and 2nd for truck & loaded RV. Though the dates are different, everything for the truck was the same on both weighs: full tank of diesel fuel, full tank of DEF, empty bed except for the 5th wheel hitch, and nothing in the truck cab but me.
Truck + Loaded RV Ticket:
So my truck weighs 9,500 lbs... and my truck + RV weighs 25,420.
Since my truck's GCWR is 33,000 lbs, and I only gross 25,420... I'm good to go there.
Now, what does my loaded RV weigh by itself?
25,420 (gross truck & RV)
-9,500 (truck weight)
15,920 (loaded RV weight)
So my loaded RV weighs 15,920... which is under the 16,500 GVWR, so I'm good there.
Those are really the only numbers I really care about. But if I wanted to know my pin weight, well that seems pretty simple to figure too:
Since the rear axle weight on the truck by itself weighed 3960... and the same axle weighed 6800 when the RV was hitched...
2,840 (pin weight)
If I want to know that percentage...
2,840 (pin weight) / 15,920 (RV weight) = 17.8% which is within the recommended 15% - 20% range.