You should not have any reason for A/C at night to sleep. It is somewhat high elevation & it will most likely cool off at night, perhaps needing an extra blanket or two. Maybe a little heat to warm it up in the morning at times.
It could be warm, but it could be cool durning the day. Depends on what the current weather might bring. A high pressure, it could be warm, even hot. Low pressure, could be cool, even cold. If rain happens, it will be much cooler.
What I have on
:)Big Bend Area. We really enjoyed Balmorhea State Park & checking out
Fort Davis Nat'l Historic Site. These are north of Big Bend & of Alpine. Balmorhea is at a lower elevation, Fort Davis a high elevation.
Depending on your camping style & preference as well as climate conditions when there, you could choose which campground you want to stay once you get there. I see no need for reservations. Can't see it being too full at that time.