I have a 1998 chev 1500 4x4 with the good ol' 350 that gets about 11 mpg towing the tractor and has 147,000 miles. I equipped my pickup long ago for towing RVs and this tractor with trans temp gauge, vacuum gauge, high rated LT tires, stainless braided brake lines, etc. I tow my tractor a lot and this setup works very well. An RV of less weight tows much worse since there is greater air resistance.
I have weighed at closed weigh stations a few times and have found 6500 on the trailer tandems and 6500 on the combined truck axles. I shoot for about 10% tongue weight and this PJ trailer tows like a dream with no sway and no porpoising. The trailer weighs 2500lbs with two braked dexter 5200 lb axles for a GVWR of 10,400 and a cargo capacity of just shy of 4 tons. The trailer deck measures 18' long including 2' dovetail. Slide in from the side ramps, recessed trucker style sealed lighting, and removable diamond plate fenders. I use a 10k# rated WD hitch, a 3/8" chain and binder at each corner, and carry the spare trailer tire in the pickup until I can figure out a way to mount it to the trailer. The trailer tires are big and E-rated.
I bought this trailer with a decent safety margin and room to grow. The 7000 lb GVWR version was much cheaper but would have been nearly maxed out. I am very proud of this trailer and have yet to find a fault.