We are around 25k GCWR. 5th wheel. No issues normally, though I just blew my head gaskets thanks to a leaking EGR...

Anyway, we have 35k miles on the combo, no issues until now. Find a hill and hold the throttle to the floor if its not tuned. It will do it.
Get Gauges.

 

My fifth wheel trailer weighs 10600 dry, so with cargo, generators, etc I'd guess I'm usually around 12000 pounds. No issues towing this trailer. It is a 33' with 3 slides. It would be a good idea to get some gauges if you don't have them, especially EGT gauge if you are traveling in mountainous country.

 

Get a ScanGauge (or equiv ) to monitor oil vs coolant temps if you're gonna tow. This is most important with a 6.0. If you haven't studded your 6.0 and are going to tow close to the max weight it will be a matter of time before you'll blow the head gaskets. I was lucky and mine blew while still under warranty. I had it studded then. Not a WHISKER of a problem since! Been about 8 or so years with studs!

Tow a 2007 29-5 Arctic Fox 5th wheel -- about 12k loaded.