Previous owner of my truck took off noisy lugged tires and put on a compromise that's quieter but still has lugs. Don't know if he changed mileage and there's plenty of tread on them, so I won't be finding out soon either.
I'm skeptical about the M+S rating because the relatively smooth tires on my previous truck had that rating but they were not ground-grabbers. I really noticed the difference when I put in limited slip differential.
Christi, if your current tires do not have an R in the number (indicating radial), then putting radials on might improve your mileage slightly, but probably not enough to justify new tires. OTOH, if you were going to put new tires on anyway, consult with Les as to what they recommend.
Usually, one wants the stiffest tires for towing, but in your case you have a small 5W so sway is a very minor issue IMHO.