I have seen some people 'Boondocking' at South Padre Island. Far end of the Island on the public beach area. Be careful not to get 'stuck' in the beach sand.
there is a full hook up campground on South Padre, but it is in the higher range of cost.
2013 Forest River Salem Hemisphere SBT312QBUD
2001 CrewCab F-250 7.3 PowerStroke Diesel
SuperChip, BTS transmission, 6.0 Trans Cooler