Can I hang cabinets and shelves?

If by "FEMA" trailer you mean those surplused by the Government, you should keep in mind that despite their looks/wheels they're often NOT equipped to function as "travel trailers". Many were designed to be hauled to a location and immediately hooked up to basic household utilities. In many cases there are no holding tanks, 12 volt power systems, and/or other things one would expect on a recreational trailer. If they're lacking, having to retrofit those basics may be problematic and very likely will be expensive.

Just something to bear in mind when you're shopping!