I think you'll be fine since you won't be climbing anything too steep. Around here, with some of our 7% grades, it might cause you some grief, but as long as you have a trani cooler, synthetics in the gear housings, and some patience, everything should work out. I'm pretty sure that you'll see a significant drop in fuel mileage too, probably somewhere around 10-12mpg, but if you're okay with these things, it certainly will be less expensive then trading up to a diesel. The 3.92's will certainly help get things rolling, I think the only thing i would consider (and that is if the bed shows some sagging going on) is to maybe put some airbags to help carry the weight and relieve some of the strain on the u-joints. The electric brakes on the 5th wheel will certainly help slow things down too. It sounds like you have the right combination on the truck, being 2 wheel drive, lower gears, and a decent sized engine. Good luck and hope you find it enjoyable.
Where we've been:
2000 Rockwood Roo M-18 Hybrid
2006 Dodge Ram 2500 Auto 5.9 Diesel
Gone but not forgotten -1986 Rockwood 1003 PUP