I just yesterday added a ceiling fan in my boy's room where only a plastic light box existed. The key was a hardware product called "Saf-T-Brace" by Westinghouse. I picked it up at Home depot for about $15.00. One thing you'll realize if you happen to try this product is that the plastic spacers on the ends of the bars were designed to work with 2x4s, which as far as I know are not used for ceiling joists in mobile homes. Therefore I flipped the spacers upside-down so the end "teeth" could grip the wood.

Now my boy has a 5-blade,36" fan hanging in his room. Naturally there was only 2-wire existing as switch-leg so fan and light operate off of one switch.