I wish you great good luck in finding your campground in San Diego...and perhaps I can help.
I have researched and stayed at most of the campground parks in San Diego County over the last 35 years of traveling back and forth to my home in southern Baja. (Baja Sur)
One of my favorite places is at Guajome County Park a few miles east of Oceanside. About $28 per night. Nicely wooded with a lake, plus lots of wildlife while being close to shopping and the beach...about 6 miles. Unfortunately, you cannot stay for a whole month...and most likely never thru a weekend, as those are reserved well in advance. I've camped there about 20 times, but only for up to 4-5 nights at a time...then had to leave for the weekend and come back late Sunday afternoons. Yes, kind of unhandy.
I did try Vacationers RV Park in El Cajon for a month...full hookups, including Wi-Fi 7 cable for $645 per month. (The best monthly deal I could find with all amenities.) Right next to I-8 freeway. Also stored a RV there for $5 per day while I was in Mexico.
I've also stayed at Santee Lakes RV Park many times, but never for more than a week at a time. About $38 per night with full hookups. It's very nice and scenery-filled. Best to get a waterfront site. Good time feeding the ducks (unless they come thru your open door...they are not housebroken...ugh!)
Portrero County Park near Tecate (east of San Diego about 35 miles on Hwy 94) is very nice...and lots more reasonable than San Diego RV parks. $25 per night last time there. Huge oak trees and pleasant walks. Quite a drive back into San Diego for the beach & shopping, but we've done it and were quite satisfied to be 'out in the country'.
Have also stayed at Campland On The Bay, Chula Vista Marina, KOA, etc. $$$
Happy Trails