The beach access road at Oceano does have a nice entrance/exit that takes you right out onto the hard sand near the water, but this leads a lot of inexperienced people to get "in too deep"...literally. Unless your motorhome is 4wd, you would be playing with fire. The vast majority of RV's camping in the sand are trailers pulled behind 4wd vehicles, and even some of them get stuck.
If you do decide to try it, all tires should be lowered in PSI for flotation. When running a 4wd truck/SUV in the sand, 12 PSI is good if you're gonna go back and forth from sand to pavement all day...8 PSI if you're just gonna stay in the sand till done playing. I am not sure whether or not these numbers would work with a motorhome though. If your towing a trailer...lower the pressure in it's tires also.
'04 GMC Sierra 2500HD, CCSB, D/A, MaxBrake, B&W Companion, Retrax Pro, AR Power Steps
2013 Rockwood 8289WS w/Diamond pkg
1969 Camaro RS/SS Convertible
Dad's old 1968 Empi Imp (recently recovered from under 21 years worth of boxes/dust)