Some points to improve trailer sway. If the race car is chained down to the frame of the car, not the wheels/ axles, it tows better. Chains on one end, not straps. The downside is that the tires take more load this way.

I foregot and left the air bags low. The lower ride alt towed better, a little low in the raer of the truck/van. Try it back to back, dont go crazy getting the back end up.
Over load springs help,without raising the ride. That's the bestway.

I use low profile tires on my open trailers. Makes a huge difference. Fuzion makes a 5 ply 225/60/15. Really nice .
I cut one coil out of the front springs on my Chev Van, use Bilstein shox.
300k of open trialer towing, about 25k with a double VW trailer of about 6500#.
Low truck, low trailer, firm up the tie downs.
This works for the open single axle trailer and the double axle , econo type trailer. Gross trailer with a SM or VW IT car and spares.
We get about 15mpg with the Tahoe( 5.7 Vortek/banks) pulling the single/SM, and 12mpg withe G20(305/ 4bbl holley) van pulling the double axle /VW.