I suppose that rule-of-thumb was written out based on what had been noticed by experienced trailer haulers. They saw that more than 20 feet of trailer on a 110" wheelbase TV tends to be more unstable. However, that rule-of-thumb doesn't take into account frontal area, nose up vs. down, tongue weight percentage, etc. I imagine though that all things being equal, it's pretty close to a good help.
That said, by that rule I should be towing no more than 22 feet. I'm at 27 feet and even without any weight distribution or sway control it's been rock steady. I think I'm benefiting from a smoother frontal area. So far though I've only towed it unloaded, with no passengers, no more than 55mph. Will see what happens come vacation time. (Yes I will use a WDH and sway control then. We'll likely keep it under 60mph since it's our first trip, and we'll be taking the side roads not interstates when possible.)
