The State of Washington Parks and Recreation has a program for resident seniors with a limited income and those with a disability. It allows camping for half-price in WA state parks if their income is less than $35,000 a year. Also free dump fees. This pass is good for life unless your income increases beyond the maximum allowed.
There is also an off season senior pass for others that costs about $30 a year.
You might want to check with your own states to see if they offer anything like these programs.