Having spent most of my summers down in the Outer Banks of NC where the sand is very soft we always put the tire pressure down to 15--never got stuck--but with a dually you can't run far on hard roads with the back tires deflated for they may rub on the side walls. also the front end of these trucks are heavy. With harder sand you maybe able to get away with a higher pressure. Always carry a shovel,a tow stap and a 2'x2' piece of plywood with you(3/4" thick).
David