Place a coir door mat at the bottom of the steps w/ a place to leave your footwear, and another mat at the top to keep sand from entering your RV via feet.
Bring at least 2 sets of bathing suits so one can dry in the morning while the other is being worn in the afternoon. Same for beach towels.
Bring lots of water resistant sunscreen and make sure it's at least 30 SPF rating. I'm partial to Coppertone's Water Babies 50 since it's readily available nationwide. Also fine for adults. Try to buy it before you get to coastal areas since it tends to cost more there.