This is the one they generally recommend the 1223 variable rate for best empty ride yet 50 percent more carrying capacity at the limit. Since it raises the back of the truck some many also add either the Bilstein 5100 or the Eibach pro truck both quality & adjustable to get both a bit of lift and your proper rake back.
https://www.sdtrucksprings.com/index.php?main_page=product_info&products_id=17042
I transport TTs commercially with my Ram 1500 ED & a 6,500 pound TT is daily fare for my truck. I didn’t want the lift I wanted better control dampening & ride so I went with a quality axle to frame air bag set like this; BTW you don’t need the fancy on board compressor any cheap plug in or cigarette lighter powered compressor that you store under your seat will quickly fill these bags even under load. For that matter you hit any free truck air or gas station air compressor.
https://timbergroveenterprises.com/old-home
Edit sorry can’t seem to make the links work. With only a 6k TT & a WDH you shouldn’t be anywhere near your 3,900 pound axle limits. Adjust your loading & WDH towards 12.0 percent tongue weight (I find it ideal) and you would only have about 720 pounds of tongue weight. This leaves room for passengers & any stuff that you have to put in the bed. BTW 560k miles transporting TTs mostly plant to dealerships. Great truck for moderate weight, great transmission, efficient engine.