Stayed at Riverside Oasis a few times years ago, but I think it's shut down. It was a great little campground, and the folks who ran it were really good, too. Pity, if it's gone.
Beyond that, Horsethief CG on the way into Canyonlands NP is nice, if you can get a spot. It's a BLM cg I believe - pit toilets and that's about it for amenities, unless it's changed in the last year and a half since we were last there. Think we paid $12/night? Quiet, dark sky spot. Maybe 50 sites?
Mike. Comments are anecdotal or personal opinions, and worth what you paid for them.
2018 (2017 Sprinter Cab Chassis) Navion24V + 2016 Wrangler JKU (sold @ ????)
2016 Sunstar 26HE, V10, 3V, 6 Speed (sold @ 4600 miles)
2002 Roadtrek C190P (sold @ 315,000kms)