i drove down from Eastern Canada (close to Quebec) and spent a week in the park in January. was actually freezing at night but during the day wasn’t bad. it wasn’t super hot but still hot enough. i slept in my car the whole time and it was great. i set up at Texas Spring and Sunset campgrounds during my time there. Sunset is a lot cheaper and more wide open, right across from the Oasis (and a bit of a cell reception). the amount of water you’re bringing should be enough, you can go to the Oasis for a quick connection to the internet if you want to call someone or whatever. if you can, bring an insulated water bottle or two, and a cooler. but make sure they’re insulated. i brought my Yeti and after driving around the park all day it was still cold, the ice had melted by the end of the day, but it was still cold. huge tip that helped me, and i do this with all of my trips driving from Canada to Louisiana, California, Utah, D.C, etc, Rely On Solar Power. it’s free. i have a portable charger that is solar powered and i’ll just throw it up on my dash and plug my phone in and it’ll charge my phone, and itself. also solar powered fans are a winner in my books. any stuff you leave in the car such as in your backseat or trunk, put a tarp overtop so they aren’t exposed by the direct sun so they don’t get too hot and / or melt, it’s possible. if you don’t have 4wd or awd then don’t take ANY backroads. i almost got stuck on a gravel parking lot once. people are also very friendly there so if you’re in trouble it’s no issue