We just got back from 4 days at the subject RV Park - there were 7 Montanas there counting us. I never caught anyone outside. A couple were set-up but looked like no-one was home the whole time (maybe they are on the Rio Grande with the bunch in the "General" forum !).
http://tresrioscamping.com/
We tried to get into the Dinosaur Valley State Park, but they were full. We drove through and it looked like it was about 75% Boy Scouts - kinda' glad we weren't sandwiched in between them !!
http://www.tpwd.state.tx.us/park/dinosaur/