I agree with the airbags or other "helper" for anything over about 500lb of tongue weight, smthng is also correct about the trailer needing to have balance. Loading is key. Also having the trailer ride level if it has multiple axles will help, especially if you only have brakes on one trailer axle. I also agree about the truck being low on horsepower in stock form for highway towing of heavy loads. I fixed that with a healthy tune, truck can hold 6th gear and can also hold higher speeds much better.
To handle my tongue weight, I have the autospring add a leafs. This is a 5500lb trailer with 800lb tongue weight, cat scale verified. You can see the extra leaf springs really help with the squat. Without helper springs, this would squat my truck much more.
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