Big Bend National Park, Texas
Big Bend National Park is located in southwest Texas, snuggled up to the Mexican border. Translation: In the summertime, Big Bend is almost unbearably hot, with temperatures above 100 degrees F (38 degrees C). In fall and winter, however, Big Bend's more moderate temperatures tempt outdoor lovers from around the world.
That means you can soak up the vast desert vistas and go hiking for miles to places like Boquillas Hot Springs and Cattail Falls without worrying about heat stroke. It's worth noting, however, that even though daytime temperatures might exceed 60 degrees F (15 degrees C), the chill of the night can quickly plunge into the 30s (source: NPS).
Unlike parks in northern climes, the campgrounds in Big Bend are open year-round. That means you'll typically be able to find a spot to park an RV or pitch a tent. Be aware, however, that the days leading up to and following Christmas and the New Year are often exceedingly popular, to the point that all front-country spots are booked (source: Thomas).