This topic is frequently discussed in the RV world via 100s of lengthy threads over the years. The consensus is non-commercial = no CDL required. Many of the larger, heavier 5th wheels and tow vehicles (including mine) are over the 26,001# limit. These heavier trailers were often towed with MDTs or HDTs (at least until the emergence of the Super Duallies in the last 3-4 years rated to tow +30,000#). Yet nobody on these 100s of threads can cite a single example of anyone ever being written up or even warned by law enforcement for a violation.
From a practical standpoint, it is concerning that anyone can walk into a dealership and purchase either a large diesel pusher motor coach or large, heavy 5th wheel and proceed to drive across country -- with no previous experience other than driving a small car around
So never assume the guy driving the ginormous RV rig next to you and a few feet away at 70MPH has any clue what he's doing or has ever pulled even a small utility trailer, because they most likely have not!