We actually own a lot in Retama Village, just up the street from where Doug and Judy worked...there are over 500 RV parks in the RGV. We personally like the upper valley much better than the lower valley...the Mission/McAllen area is much nicer!
There should be lots of workcamping opportunities...right around the corner from us is Seven Oaks...you might try there. Also up the street are Bentsen Grove, Green Gate, Americana, just to name a few...also Mission West, Circle T...the list is almost endless...Good Luck!