dave I guess there's a feel to this kind of resolve. Any of us could give you a number (probably different numbers), but your goal is: not too low so as to flex the sidewall excessively, especially with any speed, or so low as to lose the bead (wheel to tire) air seal.
The starting goial is of course to air down from a flat hard road surface to allow the tire to flex and form around the softer, possible uneven surface (as the sand beneath the tire shifts it becomes the uneven so to speak).
If you have a compressor, experiment until you can answer this question for others. Drop it by 10 psi several times and see what works. Don't go so far as to need a tire repair shop in addition to your compressor.
The correct answer is based on extreme subjectivity (tire size and load rating, weight of truck and camper, moisture of sand, looseness of sand, your driving skill level and experience, etc.)