If you are looking at Marin or Sonoma (S.F. North Bay), there are quite a few Mobile home parks, both for seniors (55+) or under.
A couple of things to know - Mobil home park usually charge a berth fee (rental fee for the space which is like a parking free for the site the Mobile home resits The feel is usually much higher than a condo HOA fee.
In addition, mobile home is treated as personal property when you try to go and get a loan, so it will take extra efforts to find a lender who loan you the money with reasonable rate, so be careful of that. .
I am assuming by your profile that you are looking in Charlotte NC. If that is the case you really need to contact a realtor from that area. Possibly Realtors in here from Charlotte can give you a hand. If you are looking for Florida I can give you some wonderful information. Let me know