I agree that you should always have an experienced real estate attorney on hand during any real estate purchase.
Are you asking whether or not an oil tank is a legal requirement in the sale of a mobile home? Does the mobile home have an oil-burning heating unit? Or is it heated with a propane unit? Is there a heating system at all?
Whether you are buying just the mobile home (not the land) or a mobile home on land I recommend that you bring along a licensed home inspector who has experience in mobile homes. S/he will be able to help you analyze the heating/cooling and other systems of the mobile home. Also should a buried tank be an issue s/he may help you locate any tank that may be buried (or connect you with a reputable oil tank abatement company.) Licensed home inspectors can be found at the New York Department of State Division of Licensing Services: http://www.dos.ny.gov/licensing/homeinspect/hinspector.html
Best of luck!