If as you say, these are mobile home style windows, then I am assuming that they are aluminum, with about a gazillion screws attached to the nailing flange.
If this is the case, I would remove the inside trim and measure the opening and also the depth of the jam. The window jam may only be about 2 1/2 " so you would need to know this to be able to order the right size. The new windows should have insulated glass.
Unless you plan on completly changing the size of the windows, this is what I would do with the information you provided.