Nice thing about Missouri (and Arkansas) is there are a lot of nice campgrounds administered by the Army Corps of Engineers. They are dirt cheap ($14-18/night) and almost all have electricity and are on a lake/reservoir. And since they're federal, you can use your senior pass to cut the price in half. Most of the Arkansas campgrounds are lightly used - even weekends. And there are so many in a small area you can usually find a spot.
Southern Iowa has quite a few Army Corps campgrounds too. But they fill up on weekends. And most of the land is agricultural with little public land so boondocking locations are hard to find.
This guy does lots of reviews of dispersed camping in Arkansas, maybe Missouri too:
