"I will also be building a gun range in my back yard so the sound of gunfire may deter thieves."

That's a two-sided sword sort of thing. It will also let everybody in the area know you are a gun enthusiast and probably have a lot of guns in your home. It won't take long to figure out when nobody's home! I would invest in a quality gun safe.

I would think a double-wide would be hard to really harden and secure, but I'm no expert. There has to be a speciality website that offers suggestions on security for manufactured homes.