Reserve America has been a very convent resource for us. On stop shop for most state parks. Makes planning a vacation much easier. The fee does vary from state to state. Looking thru my past from last year and future reservation the fees have been or are -
Virginia $5
Ohio $8
Wisconsin $9.65
Kanas $3
Illinois $5
Florida $0
Maryland $13.68
New York $9
Did a drive up no reservation once in Virginia. Still paid the fee. If some states are absorbing the cost, adding it into the nightly rental or what ever I don't know. My no fee Florida reservation was this past November over Thanksgiving for a P/W site on the beach for $24 a night while Maryland was in July after the 4th Holiday no hook up site on the beach for $29 a night. Even with the fee still costs less than a play golf cart derby at a pack them in commercial CG.
I also use Receration.Gov for COE CGs, tours and such. There's a line item for a fee but haven't paid one yet. The tow sites look and feel very much the same.