Mostly googling.
Campendium.com is pretty good.
If we are on the move, we use Passport America for the savings more than finding. PPA isn't as good when staying somewhere more than a night or two (we'll still check but usually they have limits)
We are traveling longer term mostly, so we are quite flexible on where we stop. If it was a week or two vacation, we would place more emphasis on prime location as you have limited time, so driving from a 1/2 hour away to see a national park or other site may be less acceptable and in the big picture represent a small portion of the trip cost.
Tammy & Mike
Ford F250 V10
2021 Gray Wolf
Gemini Catamaran 34'
Full Time spliting time between boat and RV