One thing to note is that not only heavier weights require the dually, but the TALLer the TC (higher CG) the more important the stability that the width of the dual rear wheels provide.
Some people can be comfortable with a lot of side to side sway, but not too many......
Side note caution:
If your running TALL with the blessing of a solid tailwind at your heels on a divided highway,
be sure to be real careful/conservative on your cloverleaf speeds. The side of the TC is a BIG sail in a wind, and if you add significant centrifugal force to a high CG plus wind...... It can ruin your day..... No mater dually or SRW.