Green Tree Mortgage (out of New Jersey) has been the only non-local lender we could go to with Pre-1976 Mobiles; but I think they too are having their own problems right now. They would obtain about 1½% to 2¾% above the prevailing 30 Year Rates.
I have also had good success with local banks who know the local property and know the Prospective Buyer; but they have to be able to consider keeping a loan for one of these Oldies in their own portfolio; which is necessary because there is NO Secondary Market for such loans.
Plan on at least a 20% Down Payment and a premium on the Interest Rate. Because it's a Double-Wide, I hope it sits on a concrete pad (not gravel); otherwise, I don't think anyone will want to loan on it - the two sides start moving independently over time, and distort the unit.
I have had people go and get construction loans on properties described like yours where, the Trailer was just going to be used as a Temporary Job Site Office while something more substantial was being built on the parcel, and the site improvements (water, sewer, driveway, transformer, and landscaping) was being recycled for the New Building.