Re: Moving sailboat to Savannah from Seattle...
We have an older F450 (2002) that we use to tow a a fifth-wheel trailer and a large power boat, and some of our sailboats. Despite the big, older 7.3 liter engine, I still get not-too-awful fuel mileage of around 9 mpg or occasionally better when towing (taking it easy and not in a rush).
One idea might be finding a power boat trailer of adequate beefiness and getting a welding shop to customize it into a sailboat trailer ... add keel pad, side supports, guides, maybe a ladder on the side, etc. and there you have a nice sailboat trailer.
If you'll be taking the southern route, give me a message and we'll keep an eye out for you passing through Arizona and New Mexico.
I assume you'll have something like AAA plus RV, Boat US trailer towing coverage, etc., just in case.