jsimpson, sorry about the trouble, and I don't have any input on the sound you said the engine made under load. I 'd adjust the valves if you haven't already.
It does sound like you've put a nice package together. I considered going the same route you've taken with a cabover camper and a bumper pull trailer, however we sometimes pull 3 horses so wound up with a 8' wide aluminum slant that we ordered with a 3 foot longer shortwall and a pass thru door to the back. It's worked out so far.
On the exhaust, I would not go with streight exhaust as it gets loud when the truck is working. Just crusing, not objectable at all. If you keep HP under 300, I think a very workable system is to remove the cat, and stock muffler and add a Walker straight through 3 or 4" muffler.
Also, I just added the 3K GSK from Piers and highly suggest this low buck mod. One of the best returns on investment I've made. A #8 plate, 3K GSK, low flow air filter, BHAF of other and the above exhaust mods and you'll have a nice tight drivable rig without spending a ton of money. Oh, gauges as well.
Good Luck, Ronnie
PS your fuel mileage is of course dependant on driving style as much as anything, but 13 sounds about right considering the air your pushing with the camper and all.