We tow a 6K# aluminum boat on a 2500# trailer (total weight right around 8.5K) and our 2520XL on a 2500# trailer with an '01 Ford F-350 7.3L PSD. The truck is a turd, its had the engine replaced, the oil pump done twice, real main seal blew at 5K miles, etc, but it handles it pretty well. Going up the Berkshires in Western Mass, we can keep 60mph. I could probably get it to 65, but I tend to take it easy.
The biggest thing is to NEVER tow a heavy trailer in Overdrive uphill. Trucks have an OD switch or 3rd gear setting in the automatic tranny for a reason. If the load is light enough, or the road is flat enough, the tranny won't have to hunt around for gears. Otherwise, it'll jump from OD to 3rd, reengage lockup when it gets back up to speed, then shift up. Repeat 20 times/hill, and you're wearing the living daylights out of the OD and lockup packs. When you get to a hill, just take it out of OD, and let the engine do what its designed to do: Work.