Crested Caracara used to be a South Texas bird, but they have really increased their range to the north over the past thirty years. They've gotten pretty common up into Central Texas and beyond.
We used to camp at Bensen Rio-Grande State Park, but they closed the campground due to impacts on the habitat and converted it to a day-use only World Birding Center. Great place to bird, but no RV camping.
If you get down to the Harlingen area again, you might look into Adolph Thomae Jr. County Park near Arroyo City. This small park sits right on the Arroyo Colorado out east of Harlingen and is a very good birding park. Or if you want to stay on the south tip of South Padre Island, try Isla Blanca County Park. It is much larger and more "commercial", but will get you in the middle of another great birding area!
Mark