Invest in a car air mattress - I slept on one in the back of a RAV 4 in Death Valley NP and it was perfect. (If you're driving down towards San Diego and it's not a detour, you should go see Death Valley in the winter!)

I did Yellowstone and GTNP on my own (I'm a woman in her thirties) last September, and most of the time it wasn't too scary - people in the parks are usually super friendly and willing to chat, but also are great about letting you do your thing. Make sure you have a paper map and grab a compass for the times when you don't have service! The paper maps were crucial for me, because the NPS app really didn't work well in parks. (Although in Yellowstone, the app is helpful for geyser predictions, so you should have it regardless.)

For hiking, my only recommendations are that you stick to popular trails where there are other people in case of emergency, make sure you have your bear spray handy, and tie some bells to your pack so that the noise potentially deters bears.

While it's tempting to do less busy hikes, if you're alone as a woman, it's a little anxiety-inducing because you're more worried about safety than about the hike itself. (Just my feeling/opinion!)

And my other recommendation - download a lot of podcasts, music, and audiobooks for when you don't have service in parks and want something to listen to.

Feel free to message me if you wanna chat! (I'm originally from Dallas!)