optimal 5er carrying capacity

First off, that 'dry weight' does not include water or propane. This is most likely why he believes in the ranges he does. The part that I don't quite go along with is that 5ers typically weigh a lot more than regular TTs. More slides, much bigger, etc. For something that much larger & heavier, a lower percentage shouldn't mean much - as long as you've sufficient cargo capacity AFTER filling the water and propane tanks. And, IMHO, any large 5er with under 3k capacity isn't one I'd want.