Bob's truck is typical of the bunch. That's pretty much how they all are, although that's pretty decent towing mileage. It would be interesting to know if the truck has 4.10:1 gears or 3.73:1 gears.
I think if you have a relatively light trailer that doesn't catch a ton of wind, the 3.73:1 will be fine, even with the 5 speed. I bought mine with intentions of a pretty good sized trailer that I'm looking at buying, I don't think the 3.73:1 gears will work with a 5 speed for a really big trailer, I think at around 28', you'll have enough weight and catch enough wind to make it difficult to keep the RPM down. They might not work at all for a really big trailer, it might not stay in 6th gear and locked up. That's the big deal for fuel economy, the truck needs to stay in the highest gear as much as possible, with the converter locked up. It is better to be in the highest gear and locked up with the 4.10 gear than be in a lower gear unlocked with a 3.73. It may be that for a bigger trailer, the 19.5" wheels and tires and a 4.10 are the ticket, that's why mine is set up that way.
The transmission cooler leak is not as common as the oil cooler leak on the earlier trucks. Most of the earlier trucks I've seen have had to have at least o-rings in the oil cooler lines, if not the lines replaced.
As I stated above, the 2006 and up trucks are the best, most have no EGR, they have the 6 speed Allison, and some other problems were resolved.
To go cheap and still get some improvement, with any year 8.1L, I think a true dual 2-1/2" exhaust with no catalytic converters, a 180 degree thermostat, and a really good tune will put you in a good place. With no cats, you can turn off the logic that adds fuel to keep them hot, the big block prefers to be cool, it can have more timing that way, which makes it happy, and the low restriction exhaust will reduce pumping losses. You'll see power and fuel economy without spending a ton, and it should do really well for a 24' or so trailer. A good shop should be able to do the tune and a dyno run or two for $250 or so. The thermostat is less than $20, and takes 15 minutes. With the exhaust, you have to be able to pass emissions without the cats, and take them off yourself. Fortunately, my GVWR exempts me from emissions here.
