I used a combination of all techniques on my cross country ride last year. Most of the time, it was possible to roll into these little towns and get permission to flop in the town park, fairgrounds, etc. As mentioned, the ACA maps are excellent about pointing out options. Lots of times, I got taken in by very nice locals, and this was one of the best things about my ride--meeting great people who are so generous. I wasn't in Muscatine, Iowa, more than five minutes before I had a place to stay--amazing.
If I didn't have a warmshowers host(check out couchsurfing too!), I'd roll into town and hit the library first. You can usually find out everything you need to know there. I simply could not afford to pay for a place to flop every night--nor did I want to. I camped under a cell tower in Maine, behind a utility shed in Kansas, in the woods in NY, CO, NM and elsewhere. The thrill and challenge of finding a place to camp provided a real spice to the journey--and often the best camping. I HATE KOA's with a special passion. Besides being very expensive (about $25/night out west), they are freakin' noisy so close to freeways and such. The only good one I've stayed in was in Mt. Shasta, CA.--very nice with great views of the mountain.
Be flexible and creative. It's all part of the adventure.