The down payment would vary based on your financial situation and the institution financing it for you. I would guess that 10% down would be a safe figure. But, again, this is going to vary based on a lot of different things. I would recommend talking to a lender who lends on mobile homes to find out exactly. You could try Altana, Valley Federal Credit Union and maybe Fist Interstate Bank.
I have seen the down payment as high as 20% with approx. 10% annual interest. Contact a local lender to see what they can do for you on mobile homes. You may be better off trying to acquire a home in which you can reduce your mortgage payment and down payment to a much better rate which will give you more buying power.