My TV is a 2007 Dodge RAM, Crew Cab SB, SRW with the 5.9L engine. I added a PacBrake and Smarty Jr, ISSPRO gauges, aftermarket covers for the AT and rear diff to enhance cooling. I tow a 33' Excel fifth wheel (15-16K) and also an equipment trailer, which loaded with a tractor and other equipment is usually about 14-15K.
No problems whatsoever towing - very stable, even with cross winds. I tow in the mountains in Colorado and have been on many steep grades. I would not do it without the exhaust brake, which is an enormous help (even when not towing), however the exhaust brake alone is not sufficient on steep mountain grades; it is essential that the trailer brakes be in good shape as well. Given the weight of the fifth wheel I went with disc brakes on the trailer.
I use my truck mostly for other things and only tow heavy occasionally. Set up the way I have it, it is a great all around truck. If I were towing more often I would go with 19.5s on the truck (I have them on the Excel), as I am at the rear axle limit on the truck with my current set up. I have air bags but surprisingly did not need them for the fifth wheel, the truck drops only 1.5" or so in the rear when loaded.