That rule of thumb sounds like it was made up by someone who didn't want to or couldn't do the math.
Any vehicle should be fine up to it's rated limit. Sometimes this includes the frontal area of the trailer, not just the weight. Also, particularly with 1500 series vehicles it's difficult to actually tow the rated weight RV because you run out of payload before hitting the maximum tow rating.
If you're talking about heavy duty diesel vehicles that's another story. Many of them can quite safely tow well over their ratings. Professional "hot shot" truck drivers do it all the time. Also farmers and contractors often go well over the ratings. I think my '05 Duramax is rated for about 22,000 GCWR. It does quite well at 24,000. That's about the most I've ever had need to tow/ haul.