On the way to Canyonlands, we've seen a lot of boondockers, but go to
Campendium and enter in Moab - this link will go straight there - and you will see numerous boondocking areas listed at $0 cost with other various RV campgrounds. If you've never been on Campendium, sign up, because it's free and super full of information. It's my go to area to planning a trip. They also have 360 videos of campgrounds and dispersed (boondocking) areas on YouTube. Sign up for that too.
For some reason I was thinking this was near Moab, but looks like it's near Sevier. Same thing with Campendium, plug in that town and see what comes up there.