I'm no expert but the most rattlers I've ever encountered were on a hillock just NW of Big Bend park boundary. Now this was height of summer in 110 degree heat and I foolishly was seeking shade and shelter along a limestone shelf after a long hike. One buzz was followed by another and soon I seemed to be sharing the blocky outcrop with dozens of them. They would certainly have attracted a curious dog. Marfa might be the closest vet which might be too far.
The amount of venom injected varies quite a bit on snake age and mood at the time with the older snakes husbanding their venom supply a bit more in defensive situations. They need that to secure meals.
Big Bend scorpions I have no experience with.
If you take your dog I would try to keep her leashed or supervised as much as practical.