and here is what Mario said:
The goal in sand is to "float" on top of the sand and eliminate wheel spin. When you drive over sand and leave a perfect imprint of your tire tread, you have reached that goal. Be mindful that different types of sand require different pressures i.e. coarse hard packed moisture laden sand on a beach vs. fine almost silt like sand that would be near a dry lake bed. We did some experimentation a few years ago in some dune sand in Arizona in order to find the "traction sweet spot" for my Comanche and trailer. The truck weighed in at 4250 lbs and the trailer at 1400 lbs. Tire size on the truck and trailer was 285/75R16 E rated, BFG AT. We made several attempts at climbing the small dune, first at "dirt road pressure" 20 psi on the truck and 15 psi on the trailer. We were stuck instantly due to wheel spin. We lowered the truck down to 15 psi and left the trailer at 15 psi. We got a little further up the dune but still bogged down and got stuck. We made several more attempts, gradually reducing pressures to find the "traction sweet spot" until finally with pressure down to 10 psi front, 12 psi rear and 8 psi on the trailer, we climbed the dune without heavy throttle and leaving a perfect imprint of the tires in the sand all the way up.
If you have the opportunity, I would suggest going through a similar exercise with your tow rig and trailer to find your traction sweet spot for sand. A fun way to spend a few hours and learn a lot about your rigs capabilities.