Not all beaches are created equal.
On some the sand is packed and have decent driving trails that take you in and out to a parking area.
On most beaches I have visited the soft area like dunes will not support an RV and "you will" get stuck if you attempt to drive on them.
Generally the further away from the water the softer the sand. I no longer care to drive on the beach due to the sand getting into everything and damage from salt water over time and several trips on the sand you will have under carriage corrosion.