US80 ??

Well, actually, it doesn't go all the way across anymore.
1. What's more, it goes thru lots of cities, large and small, such as Macon, Montgomery, Jackson, Shreveport, Dallas, etc. etc. Not exactly a nice biking route.
2. When, exactly, are you planning to do this? There's a difference between hot and humid in Hawaii and hot and humid in July in northern Louisiana - - like about 20 degrees and no ocean breeze. Leaving west from the Georgia coast would be best in late fall.
3. If you are starting near Savannah, consider Fort McAllister rather than Tybee Island. Tybee is a zoo and the beach road is a nightmare.
4. If you want to do something close to US 80 - consider small state routes south of Macon crossing the Chattahoochie at Fort Gaines thru Camden, AL and Raleigh, MS crossing the Mississippi at Natchez. You can connect up with the Adventure Cycling Southern Tier or I'll be glad to discuss options in the West which I know quite well.

Best - J