Yes, Mission and McAllen are huge cities with the normal big-city problems including lots of traffic. However, I wouldn't say that the whole area is unsafe. It's just like any other large city would be. You'll get to know the surroundings.
Without knowing what parks you were offered work, it's hard to say about the immediate area and how crammed the RV sites are. Perhaps you can view them on Google Earth and see if they would fit your requirements.
There are many 'natural areas' to explore but you'd probably have to drive to them. Good luck!