After 20 years of towing 5th wheels and not being one to cheap out on tires (tread separation failures are too expensive and time consuming to repair), my preferences are the following all steel construction radials:
6000 lb axles - Michelin XPS Rib rated 3042 lbs @ 80 PSIG in size LT235/85R16E
7000 lb axles - Goodyear G614 RST rated 3750 lbs @ 110 PSIG in size LT235/85R16G
8000 lb axles - Michelin XTA or Goodyear G114 rated 4805 lbs @ 120 PSIG in size 215/75R17.5
I've used all of the tires listed above without any problems or failures.
Rusty