I'm surprised no one has mentioned Boondockers Welcome : Boondockers Welcome | Be My Guest RV Parking
You can stay with fellow RVers for free, provided you make arrangements with them in advance. There are way more people participating in Boondockers Welcome vs. the Harvest Host program, I'm sure.
There is a fee to join, but it's modest, if you are willing to provide other members with a place to boondock at your own home: $20/yr or $15/yr if you sign up for 3 years.
Also, check out the owner's series of very helpful eBooks on places to stay for free or very little when traveling in the US Southwest. The series is called "The Frugal Shunpiker's Guide to xxxxx". I've used her recommendations several times on my recent camping trips to Utah and Arizona.
The author and her husband make months' long excursions from Ontario to the US every year and usually pay an insanely low amount for camping fees, as they scout out their own free campsites. Granted, they are traveling in a conversion van and not hauling a TT, but a Roadtrek is not what you would call an off-road vehicle.