Depends on the sand. Some beaches are hard pack and you drive right on, others are sugar sand that can swallow you up quickly. Our beaches or on ramps are soft sugar sand and the park personal do not provide proper instruction to the people coming on. Especially the day trippers. They invariably don't air down or not enough and use the gas pedal to rototill the on ramps making it difficult for the next people. The beginning of the season when the beaches havent had much use I go to 20psi. Late summer 13 psi in the front and 15 psi in the rear. This mind you also depends on the tires. Some rvers who are on alot have summer tires and winter tires. The summer tires are big balloons and they just float on top with minimal effort.
So you need to see what conditions you are attempting and dont be afraid to ask someone else who is airing down in the same place. I ask people coming on and off to see whats up.
Happy trails