I suggest you first read Joe Lstiburek's book for your climate available at http://www.buildingscience.com

Read on the Top 10 Stupid Things Builders Do in the South. See the part about vinyl wallpaper? You have a vapor barrier trapping moisture inside your home. Your bathroom is a terrarium! Remove that wallpaper.

-Install proper powered ventilation that exhausts to the outdoors
-Balance the house including makeup air for the "fart fan". Follow ASHRAE 62.2-2004
-Do NOT install a return in the bathroom. That is polluted air you want exhausted--not shared with the house
-Monitor your indoor Rh% and control as needed
-Get the water away from your house. I install gutters with the leads run well away from the house even if you feel that sandy soil can handle it. If crawl space, install a vapor barrier. Seal off vents to crawl space
-Take cooler showers for shorter periods. Take Navy showers (rinse,stop water, lather, rinse, done)
-Install functional exhaust fans over stove and other bathrooms.

The last thing you want to do is to share that funky bathroom moisture with your ducts, even if metal lined. You'll grow goobers in there and soon your house will smell like gym socks.