KOA's are fairly reliable. Keep in mind - a KOA journey is designed for an overnight, while a KOA Holiday is designed for a longer stay.
If using campgrounds, I look hard at the reviews of all the campgrounds in the area that I want to stop.
If I were making that trip at the beginning of my RVing life, I'd do the following:
1. Plan more days. The schedule you have will be no fun. I'd plan for no more than 3-500 miles per day, especially as a newbie. Long travel days wear you out, and you will probably average 50-55 mph. And things will go wrong that you have to deal with. Maybe minor things, but that's just the nature of the beast.
2. I'd pick up a cheap 3500-4500 watt inverter generator. That's really all you need to start boondocking. It opens up so many possibilities. When we started, we had a generator and kept about 30 gallons of water in our fresh water tank, so we could boondock if needed, and dumped our waste tanks when we hit the next campground. 30 gallons will easily last 2-3 days.
3. Join Harvest Hosts- lots of nice overnight spots. We prefer this program for our overnights to parking lots.
4. Pay attention to any casinos on the way. They are also good overnights.
5. Crossing Texas - it's a long trip and there is nothing in west Texas. Just pick a campground that fits your travel plans and move on.
6. Key West - better have reservations, and prepare for sticker shock. If you are eligible for military campgrounds, they have a great one that is biking distance to downtown.