What I have on the
RGV.
Can't help on the RV Park Question, but for the Shrimp part nearer to the Gulf would work best, IMHO. As for Mexico, I would agree with crossing at Progreso & walking across. Not far from Harlingen/San Benito as well. Lot of PKs in both also.
I would suggest going to the Laguna Atascosa Nat'l Wildlife Refuge out from Rio Hondo. I'd want to stay out there some as well. "Overnight camping and a boat ramp are available in the Adolph Thomae, Jr. County Park, located within the refuge near Arroyo City."
Hope some of the old post will be of help.