We've made the trip from St Louis to Quartzsite on several occasions leaving the day after Christmas. We do our best to get as far south as we can before turning west. Just before our first trip, a friend that lives in Albuquerque warned me to stay off I-40. Best advice I ever took. As we were over-nighting in Las Cruces, we heard on the news that I-40 was closed at Albuquerque due to a blizzard - for 3 days! We've been heading down to Little Rock then across Dallas-Fort Worth towards Abilene to get on I-20 which runs into I-10 in western Texas. If you're going to San Antonio (nothing like the River Walk), you'll be a lot farther south that I-20. One of the sights to see in Austin is the nightly emergence of bats from the Congress Street Bridge - millions of them begin their nightly search for food right at sundown. It's a party every night - the TGIF overlooks the river right at the bridge and provides a great view.
Out west the temperature is all about altitude. The lower you can get, the warmer it is. No matter where you go, the sights are always spectacular! I enjoy every mile no matter where I am - it's a beautiful country.
If you're going to Organ Pipe, I might suggest staying at Holt's Shell service station and campground in Gila Bend, AZ just north of Organ Pipe. They offer a Pass Port America rate of less than $13/night (about $200/month) for full hook-ups. No frills but I don't do boondocking unless I have to. Be SURE to stop by Quartzsite, AZ the last full week of January when the BIG TENT is in operation for one of the biggest and best RV shows ever. I call it the Sturgis of RVing - every RVers should go there at least once. Nothing like it anywhere.