Garmin RV 895 and Fifth Wheels

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