A 10 speed will get the job done 99% of the time for all OTR applications. 18 and 15 speed transmissions are becomming a thing of the past in the market place.
Generally, 18 speeds are only used for very heavy haul applications. These transmissions were popular when HP ranged from 250 to 350 with torque ranges from 1050 to 1250.
With the advent of higher HP higher torque engines, that many gears are entirely unnecessary.
For RV porposes, even a 10 speed is too much. The lower gears will almost never be used. For towing, 4th gear is sufficient for starting off. I use 5th gear starting off bobtail. The only time an RVer will use lower gears is for "creeping."
onezman
http://onezman.tripod.com/yourrvhaulercom