I bought my 6.0 in Oct 03. Since then I have towed my 12,750 lb 5th wheel from Oregon to Anaheim, CA and back and several trips over the coast range.
No problems at all with the truck except for surge, which a flash fixed.
I had a 2001 7.3 Lariat that I towed the same trailer with. It towed fine, but needed more power. Right or wrong, after larger exhaust, 4 bank chip and a more open intake, it still did not have enough for towing heavy mountains beyond 40 mph.
Now granted, that the 4.10 gears help a great deal when towing up the mountains, but this 6.0 with the torqeshift works great for me.
The difference was telling while towing the Grapevine. It is a steep, long grade (I think it's 8%). I still could not get up the hill faster then 50, but, that was only cause I could not get the slow-poke drivers out of my way