I think that is referring to the height of a 5th wheel not the length. In fact, for a given length of the trailer, a 5th wheel would be shorter than a travel trailer because part of the 5th wheel is above the truck instead of behind it. Ex. A 30 ft. TT would be longer than a 30ft 5th wheel if you measured from the front bumper of the truck to the rear bumper of the trailer. It would certainly be taller though and perhaps there is not enough info on bridge clearances etc. to recommend routes for a 5er in those areas.
Could be wrong although I can't think of any other reason they would specifically mention 5th wheels.
Barney
2004 Sunnybrook Titan 30FKS TT
Hensley "Arrow" 1400# hitch (Sold)
Not towing now.
Former tow vehicles were 2016 Ram 2500 CTD, 2002 Ford F250, 7.3 PSD, 1997 Ram 2500 5.9 gas engine