I like and use: Allstays Pro, Gaia Pro, Overlander and Google maps (or Maps on iPhone). I hear Campendium is much like Allstays, I don't personally know..
As a Boondocker, I don't book ahead and it has always worked of pretty good. Timing is key, arrive near noon if attempting State or National Parks. Most times they hold back 1 or 2 sites as FCFS. Town & County Parks are usually inexpensive and some very nice. National forests are some of the best spots and you'll find several along that run.
I ran most of the Gulf Coast across this winter and found the Walmarts did not allow overnighting. They were always a great backup plan. Our favorite spot to camp was St Georges Island State Park in Florida. There is some nice free camping in Florida, but you will need to get a permit and pick you days. Most you can stay a week (or more) and being free, if you miss those dates, of well... beauty of free!