Mission has lots of places to explore...Bentsen Palm State Park, World Birding Center, World Butterfly Center. You can bike or walk all over the park.
As for the RV parks, most of the sites will be close together and as someone else stated, pretty full during the winter season.
We own at Retama Village in the Bentsen Palm Development and like the fact that it is off the main drag...US 83, and closer to the state park.
As for safety, have never felt unsafe anywhere in the area...as anywhere in the US, there are places you know to stay away from, especially after dark...use common sense and you'll have no issues.