A newer Factory Built home will actually gain in value the same as a site built home if it's built on a private parcel or in an upscale Manufactured Home Condo Conversion, Sub Division or Planned unit Development "PUD".
However, if the home was built circa early 90's or before they do tend to lose value simply because a lot of the building materials that were used then were inferior to site built homes. That stated today's Factory Built Housing is superb and in many if not most cases built to the same standards as any site built homes i.e. 2x6 exteriors walls, upgraded flooring i.e.. tile, slate, hardwood, etc, up graded window coverings, applianences, fixtures, etc.
You can even have them built and delivered stucco and tile roof ready. Additionally, you can have them designed with curvilinear details i.e. turrets, pop outs, enclaves, insets or any architectural detailing that you can incorporate into a site built home. You can even design multi-story homes today.
If you cruise up and down Topanga Blvd and do a little searcihing and ask a few neighbors you will discover a part site built and part Modular home that was built back in the early 90's. I can't remember the exact address. But when it was first brought in and crane hoisted onto the site built portion it was met with a lot of community resistance and resentment. Today it is one of the most beautiful and expensive homes on the Topanga.
I would try to avoid a rent/lease park/community unless it's really in an upscale area.
Here's a link you may want to check out: http://www.topotopanga.com.
This will give you an interesting perspective about FB homes vs trailers and mobile homes.
Another high end MH rent/lease community where you will probably never see values decline is this link: http://www.latimes.com/classified/realestate/news/la-hmw-gui
Once again and you've heard this several times I'm sure, it's all about location, location, location. There are several very pricey upscale rent/lease and even more resident owned MH communities salt and peppered all over the SoCal area.
Don't be discouraged by RE professionals who no nothing about MHs try to talk you out of it or degrade or bad mouth them. They are usually just trying to sell you a home and they don't know much about MH's so all they can do is bad mouth them.
However, do be aware that the older the MH the poorer the quality of construction and the more you'll have to put into them for repairs and remodeling. So instead of dealing with all of that find a location you like purchase an old MH, pull it out and install a new one.