I'm going to throw something into the mix here that's based on some stuff I've read, rather than stuff I've actually had personal experience with. Hopefully, some people here WITH personal experience can comment on it and correct me if any of it is wrong.
I've read that once you get beyond Load Range E tires, airing down for better traction off road - whether we are talking beach running or forest service roads - is no longer a viable option. Supposedly the much heavier sidewalls keep it from sagging and getting you the much larger footprint airing down normally gives.
I also read an article on beach running by a guy with a DRW truck & camper. He put 2" aftermarket spacers between his rear wheels, which allowed him to air down on beaches without the usual problem of the two sidewalls touching.
This leads me to think that the 19.5 inch wheels and tires are a valid solution for people that run on pavement all the time, but are less than optimal for people who take their truck and camper off pavement.
Comments, anyone?