Long wheelbase improves stability, the longer the smoother the entire rig will be on the highway, the safer the braking performance. If you don't believe it, just hitch up a trailer to a 1500/2500 pickup or a 4Runner back to back and see for yourself.
One of the reasons I never sold my Sierra 2500 HD 4x4 farm truck is because I tow, a lot. Whether it's a tractor, my FJ80, my Boston Whaler, or a small camper, I log more miles than I want to with something hitched up.
I recently towed a twin axle yard trailer with my riding mower, two dirt bikes, and some other crap in it with my 4Runner because the truck was at the farm and I didn't feel like driving to get it. The whole trailer couldn't have weighed more than 2500 pounds. At 70mph the 4Runner is all over the place. I have to be careful when I stop, crosswinds are a *****, and you can forget acceleration.
The GMC with its 6L V8 doesn't drive any differently with a small (I think 5000 pounds is small) trailer in tow. It accelerates, stops, and cruises at 70-75 comfortably. It will stampede up a hill without wheezing out of power and can bring the entire thing to a halt in case some moron in a Civic cuts in front of you and slams on the brakes--remember that long wheelbase thing?
4th gear in "S" is not like tow-haul which changes the behavior of the shifts to prevent hunting and overheating. It also makes sure that there's always some engine braking at lower speeds so you're never coasting along. The 4Runner doesn't even have a transmission temperature gauge and the transmission isn't a heavy duty unit designed for towing, nor does it have a cooler. It reminds me of the little guy who can lift as much as the big guys, but hurts himself in the process of showing off.
The 4Runner is NOT a tow vehicle. I wouldn't go "cross country towing" with it for safety, practicality, and comfort reasons. End of story. If you want to tow a camper all over the place you really should have picked another truck.