Not all my mods were initially intended to be used for towing but I ended up transporting RVs for 2. 5 years so I am glad I had installed the NV5600 and Con OFE clutch. Of course the power mods, gauges, 4" exhaust, air filter and many other mods that were already there came in handy.
-B&W goose and companion 5th wheel hitchs
-Blue Ox weight distribution hitch (didn't use bars much)
-Air bags
-Prodigy brake controller
-CB (already there before driving)
-115 gal. aux. fuel tank
-low profile tool box
-extended mud flaps
-12v power outlet at back to run air compressor for air bags (didn't get fancy with onboard system)
-Putnam class 5 receiver to replace cracking stock one
-CIPA or Bestop extendable power/heated mirrors
-cell antenna
-built bed to replace rear seat and had a custom foam with zip on cover made.
-400w power inverter to run laptop (bought laptop specific for job too)
-exhaust brake
That's all the towing specific mods I can think of right now.
Lots of mods pictures here including some of the above mentioned:
Picasa Web Albums - Mark - Truck Stuff