It's been a number of years since my Aunt and Uncle sold their Mobile Home Sales business but I can remember that 10% down was standard in the late 70's. And that was for a new one. As to negotiating the sales price with the owner, that would be up to him. If he priced it high you might be able to get him to come down. Your best bet is to have it appraised to get an idea of what it's worth and go from there. You always stand a better chance if you know what you're talking about.

If you can get a loan with ONLY 10% down you're doing great...Yes get the appraisal.People selling their mobile homes think they are like real estate ..Mobile homes are not considered an investment..Also the interest rate on mobile homes are generally higher..If it's new there is alot of negotiating to be done ...A brand new home in CA in a park selling for 152,000 has a cost to seller of maybe 90,000..If it's in an area you want to live in that's a big part of what you are paying for.
If it's not new when you get the appraisal they will tell you what the seller paid for it ...
Good-Luck