I tend to search Campendium first with the price slider slide to $0-20/nt. This is your best chance of finding RVing spots vs. car camping spots.
After that, I’ll search out the iOverlander app. You have to be more careful here as there’s a lot of vanlifers and folks car camping.
I’ll check Harvest Hosts/Boondockers Welcome. But rarely use it because if we are going to stop for just 1 night, it’s going to be a late night stop. Or we need 2-7 nights for a stay.
And then worst case, I drop back to Hipcamp and see if there are any private lot owners who have inexpensive boondocking options. This is more hit or miss, price-wise.