My parents own double wide trailer. Their central ac constantly freezes up. A "friend" had them get a 3 1/2 ton ac unit for their double wide trailer. Is this to many tons for a double wide? Is this why it keeps freezing up?

If sized properly it may be right depending on square footage, solar load, insulation etc. That said, it does sound a bit big for a DW. Make sure air filter is clean.