Richard,
Would drop directly down to Uvalde from Fredericksburg and take I90 West. Plan on staying in Del Rio and check out the area there is a lot to see, but the Alamo movie set is closed to the public. When you leave Del Rio the next quality Campground is in Alpine at the Lost Alaskan RV Park about a 3.5 hour drive. You can check out Marathon on your way though, takes about 30 minutes. When you get to Alpine you can use this as your base and see Big Bend Ranch State and National Parks, travel along 170 along the border and see Terlingua and some beautiful scenery along the border. Be sure to bring your Passport or Visa as there are Border Patrol Check Points between Marathon, Alpine, and Marfa and the border. You can also visit Fort Davis and the McDonald Observatory, Sul Ross University has a Museum and a great campus and if you hang around till February you can attend the oldest Cowboy Poetry Gathering in the country. When you leave Alpine you will be heading for Van Horn and use I10 for your return trip.
Have fun, keep your eye on the weather, this time of year there are snow storms and freezing weather along I90 and points north. This year there has already been snow and freezing rain between Del Rio and Alpine and points west, remember you are in the foot hills of the Sierra Madres Mountains and at Alpine you are at 4500 feet above sea level and at sea level near the border while you will be at 7000 feet above sea level at the McDonald Observatory.