Many many years ago I had a 1975 Tioga 22' Class C motorhome. It was on a Dodge 2WD chassis. I loved to surf fish at Pismo Beach and Guadalupe Dunes. That motorhome spent three years hauling my girlfriend and me and our friends up and down the beaches. I can only remember getting stuck in soft sand twice, and both times we were quickly yanked out by pickup trucks.
My point is, I doubt that all those Class C motorhomes you see are all 4WD. The drivers just know what they're doing. The whole "secret" is to air down your tires to 20 lbs or so. They get plenty of flotation and will only sink in the softest and deepest beach sand. Momentum helps too. Also, you want to drive just above the waterline while the tide is going out. Damp sand is like the Wal Mart parking lot, NO Problem! Of course, if you get too close to the water, well you know what happens then!
Even with those old 16.5" tires I never popped a bead in those years.
It's funny how the older I get, the crazier I was!