Your goal is a perfectly level floor of the trailer, front to rear, after
you have the trailer wet and loaded and the spring bars of your weight-distributing hitch adjusted tight. Jamie65 is on the right track, but it's a bit more complicated than that, because the hitch weight lowers the rear end of the tow vehicle.
1] Use a 4' carpenter's level and get the floor of the trailer level, front to rear.
2] Get the GVWR of the trailer off the sticker on the front of the trailer. Multiply that GVWR by 0.125 (12.5%) to get the estimated hitch weight of the wet and loaded trailer.
3] Back the pickup up close to the coupler. Lower the tailgate and have people (adults, teens, kids) with total weight of about the estimated hitch weight sit on the tailgate. Then use Jamie65's procedures to determine the amount of drop your shank/ball mount will need.
Don't even think about towing that big TT without a high-end weight-distributing hitch, such as a Reese Strait-Line, Husky Centerline, or Equalizer. Those hitches all have an adjustable shank that should have plenty of drop for your needs. But just in case you need more drop to wind up with a level trailer, they all sell adjustable shanks with more drop. For example, here is one with up to 8-5/8" drop:
Weight Dist Shank - Drop Only - 11" Long - up to 8-5/8" Drop - up to 1,400 lb tongue weight