Neville

As mentioned before, the majority of Texas land is privately owned so your options are limited. However, those options to provide some very isolated experiences.

Check out Guadalupe Mountains National Park out west. The Pine Springs camp area can be a bit crowded but you'll be by yourself more often than not in the backcountry.

Another option is Devil's River State Natural Area with the nearest town being sixty-five miles away. There's a good chance you'll be all alone there. I certainly was. This may not be what you're looking for since it's a desert environment but it's still a lovely place.