All Corps of Engineers parks offer those with Senior Pass 1/2 off fees. Often have great sites for $7-$12/night.
We use membership parks - often you can find a used membership that will help you save a lot of money. If you are on the west coast, membership parks make sense for fulltimers.
Use Passport America for 1/2 off fees at parks all over the country. Often have restrictions, so make sure you read the restrictions for each individual park.
We belong to Escapees and use their parks (and the Co-Op parks) whenever we can.
Look for city/county parks and fairgrounds.
I recommend AllStays app for smart phones for places to stay.
Barb