Your right DD. I have never heard of a 3500HD at 20K. We have 4 20ft landscape trailers. 3 at 7K and 1 at 10k. I am having another trailer made for the 10K. The 7k do end up breaking springs and bearings.I will always buy with 5K axles now. I have either GMC3500's or GMC 4500 cabover pulling these trailers. Do the math.
3500(12,000lbs) + 20ft Trailer(10,000) = 22,000
4500(15,000lbs) + 20ft Trailer(10,000) = 24,000
Both combos under CDL and Legal (26,000)
It's a no brainer for sure, get the stronger axles