If you haven't tried them, the Corps of Engineers parks are some of the best in the country. You will find them pretty much anywhere the Corps built a dam for flood control which means there are lots of them in the states with rivers that feed the Missouri and Mississippi although you will find them in many other states, also. The COE campgrounds show up on Federal recreation, camping and tour reservation information - Recreation.gov
so you can look for them that way.
Typically, the COE CG's have larger than average sites, many of them have electricity and quite a few have full hookups. Costs for full hookup sites are in the $20-25 range and most COE locations will honor the federal senior pass (although they can't sell it to you). We use them frequently and have never been disappointed.