Things you need to know:
-- where your engine makes torque and hp, (what rpm)
-- ratio of 5th gear in your transmission, (is it direct drive or over drive?)
-- what type of driving you plan to do, (highway, hills, etc)
-- your goals, (do you plan to tow the 5th wheel all of the time?, are you going for maximum fuel economy or do you want power?)
You could obtain a split rear end with faster gears, set up for highway mileage at a few hundred rpm's over peak torque in 5th gear with the High rear end ratio, (numerically lower). Then you'd always have the lower, (numerically lower) rear end ratio for towing or hills.
The split rear end is a real nice option, however it is also possible, (easy) to swap in a single speed rear end. If your 5th gear is in fact an over drive, you could set up things for towing in 4th a little over peak torque, then use the 5th for mileage.