Yeah OC has a strong signal here too. I'm guessing they have a repeater on Santiago or something. The CHP uses lowband and works much better over terrain. I can hear lots of CHP comms (39.36, 39.72, 39.44, 39.8, 39.26, 39.88, 39.6). I also can get CHP from El Centro on 42.920 and some of the LA offices, especially 42.500 Altadena and 42.120 Santa Fe Springs. Can't hear San Diego on 39.4 though... bums me out. Can you hear 39.4 at all from Perris?
Regarding San Diego I cannot hear it good enough to monitor. There is just way too much cellular noise and I have hills blocking my view. But when I go on the other side of the hill I can hear San Diego RCS North control channel (863.150). I couple times I've managed to hear the San Diego RCS South control channel (868.4125 - decoded RCS talkgroups, not Ontario), and the SDPD system. But it's just too scratchy and sporadic so I really never bother.
I used to live in SD and have folks down there that let me keep one of my scanners at their house. I feed the audio over the Internet so I can still monitor SD. I did the same type of thing to fix my Murrieta PD problem...scanner at the office

Now the only local agency I have trouble hearing is Corona (just too much static - thanks to you guessed it... terrain!).
My best catch so far in hearing Las Vegas PD from Sun City. Doesn't happen every day though!
By the way... welcome to RR! Nice to see additional folks from Riverside County on here.