Never actually camped in the area, but here's a
03/25/13 Post on the Subject.
March Air Force Base (Riverside) has a FamCamp, if you qualify to use them. Such as Retired Military. It's an Air Force Reserve Base. :C
March FamCamp seems to offer all you are asking @ $17 a night.
Just found this:
Rancho Jurupa Park - A Riverside County ParkSeveral Riverside County
Parks with camping it seems.
Park Descriptions I didn't realize how large Riverside County is. All the way to Blythe, Wow.
Might be the only one for you: "Rancho Jurupa Park is situated along the Santa Ana River and is located near charming downtown Riverside just seven miles South of Mission Boulevard on Crestmore Road. Flourishing cottonwood trees, gorgeous meadows, and two lovely 3-acre lakes surround this beautiful park."
However,
Lake Skinner Rec't Area might work as well. "Nestled in the hills of Winchester, Lake Skinner is only a few miles east of the Temecula Wineries and within easy access of Interstates 15 and 215." Oophs, After looking at a map, not so close to Riverside at all.
Good Luck with your Quest of Returning to Yellowstone.
:CDesertHawk- Las Cruces, NM USA
*2015 Lance 1985~Casita de Campo~23' 4" Tongue to Bumper, Dinette Slide
160 watt Solar Panel/GoPower! Solar Controller
*2009 White Ford F-150 Reg. Cab
Long Bed with A.R.E. Molded Fiberglass Topper
*Previously~ 2005 16' Scamp