I'm not aware of free camping in the area. The Blue Jay NFS campground in nearby Cleveland national forest is decent at right time of year. It is at 3600 ft. elevation summer generally not best time due to daytime heat and flies/gnats. Also subject to partying on wknds. It really is quite pretty up there much of the year, big oak trees and green meadow in season.
There is also camping at Wildomar OHV area. If you're just looking for a bargain Pechanga RV park on Passport America (Temecula) is a deal Sunday thru Wed nights. Free overnight in one of their parking lots. These areas get quite hot in the summer and AC is a plus. Temecula is an area where the night tends to cool off.