We love Oceanside and stay at Paradise By The Sea RV. We have stayed there twice for a month Sept/Oct. We'd be heading there now if we didn't know we had to stay home "sometime" to get the house ready to sell. Now is "sometime" & boy is it killing me.
It is not right on the beach, but it is an extremely short walk across the street. The park is well kept and there are quite a few regulars that come in the winter for long stays. A good walk to the pier where there is always something fun going on. The Thursday night food market near the pier is a weekly treat. We love to bicycle and it is perfectly situated for that. It is also a short distance from an Amtrak stop.
Speaking of Amtrak, that can be a factor staying in many of the So Cal beach spots as the tracks run right by there. It is an issue in Oceanside, but never bothered us. Pretty much the same for Carpinteria. Last March we stayed at San Elijo and said we wouldn't really want to go back again because of the train noise. We usually don't hear it much in the RV, but it sure did bother us there.