On my 2500HD Duramax short bed extended cab, I have been towing a 32.5 fiver with a GVWR of 14,000lbs. When I ordered the fiver I got the 7,000lbs axles for the oversized breaks. I use the Breaksmart break controller and in the past 2 years I have not had a problem. Earlier this year when I needed new tires I went to a LT265 75R16 for extra load rating. right after I got the truck I added 3200lb timbrens. I have made 3 trips coast to coast, using I10, I5 and this time I40 and RT99. The fiver and truck ride level, no breaking problems or pulling up 6 and 8% grades.
Last year I had the truck and then the truck and fiver weighted on a CAt scale. The truck with full fuel, myself and my wife and all the regular stuff we carry weighed, Steer Axle 4240, Drive Axle 3200 and Gross Weight 7440. With the truck the same and the fiver attached, Steer Axle 4300, Drive Axle 6220, trailer axles 12700 and Gross Weight 23240 this was on the 245 tires. I believe the sticker on the truck gives fron axle 4800, rear axle 6048 and gross of 9200 with the stock tires. The 265 tires have a rating of 3415 in single configuration, oh I am running single wheel configuration. The 05 rating for CGWR is 23,000lbs.
I do not worry about it at all.
2016 Chevrolet Silverado 2500HD Crew Cab short bed Duramax/Allison. My home, a 2006 Hitchhiker II by NUWA, 32.5' FKSBG GVWR 14000.