I can give you my experience with the '06 25RQ. I towed it with an '07 Nissan Titan King Cab 4WD. It had a 5600 lb curb weight
. The 21 is about 5200lbs. The 21 also had issues with weight
distribution contributing to sway, and the factory "fix" for it was to weld a couple of hundred pounds of ballast onto the tongue. I never really understood the 21' market... it was almost as heavy as the 25RQ, and was more cramped than the 17... the bed is a corner double that is hard to make.
Anyway I towed my 25 RQ some 20,000 miles in nine years with the Titan and came to the conclusion that the square frontal area was more important to the difficulty of towing than was the weight
. And i wouldn't have wanted any less of a truck to tow the 25RQ with. I had a 7500 lb Ford Excursion V10 when I bought the Titan, and frankly I wish I'd never sold it as my tow vehicle. I don't see any reason that the 21 would be any different at all with a mere 400 lb difference in curb weight that is completely negated if it's had the sway "fix" ballast installed.