Grocery store VIP cards....dry camping is cheaper then with hook ups.
With the Interagency pass, know as America the Beautiful or Golden Age federal CG fees are 50% off, electricity will be 3 to $5 extra except at ACOE CG's. This pass is $10 and not to be confused with the Senior Parks Pass. Saw a write on this and the reporter confused the two different passes.
Passes as previously mentioned are for 62 and older. Some state parks that have an entrance fee, TX comes to mind, get the annual if your going to visit enough to pay for it.
BLM has a lot of free camping, dry
USFS has some harder to find and watch the rules.
ACOE has some free camping but difficult to find, get their Camping with the Corp book since they are changing the web site
USFWS has some free CG's, very difficult to find.
NPS has a few free CG's along scenic byways like the Natchez Trace.
Google search is your friend, also here
https://freecampsites.net/ membership is free.