It looks like Joe has a 4WD dually and a heavy camper. All the responders have it right except Vulcan. It appears that he owns a 2WD class C; not the kind of rig I would want to take on the beach, as he has noted through his paradigm. Every one of us TC-er's will find out sooner or later where the edge is in driving on sand. So many variables, including the sand itself. Joe's determination of 18 pounds is close to what I would do. With duals, maybe only down to 20 on the rear axle, 18 in front. Here is what 20 pounds looks like pulling a not-aired-down trailer with my super singles on blowsand:
Here back on terra firma fronts @ 22 pounds, rears @ 20 pounds.
Don't even ask how this happened. Just know that I quickly pumped the tires back up using my 20 pound CO2 tank.
jefe
'01.5 Dodge 2500 4x4, CTD, Qcab, SB, NV5600, 241HD, 4.10's, Dana 70/TruTrac; Dana 80/ TruTrac, Spintec hub conversion, H.D. susp, 315/75R16's on 7.5" and 10" wide steel wheels, Vulcan big line, Warn M15K winch '98 Lance Lite 165s, 8' 6" X-cab, 200w Solar