Go ahead and finish your tiers. Put each one in its own box, then double-wrap in plastic wrap and then wrap in foil. Freeze. Travel with the boxes in ice chests sitting on dry ice. The day before you need them - or when you arrive - put the boxes in the fridge to defrost slowly. The day of, set them on the counter and let them come to room temperature before you even unwrap them. This way, any condensation will be on the wrapping and not the cake. Unwrap and stack and finish up.
My dh and I drove from Texas to California with 2 cakes, one fondant and one bc. After being frozen for a week and driving for 23 hours, I didn't expect much, but I followed the instructions exactly (originally from BlakesCakes, by the way) and they both turned out perfect. 
Last month I traveled 6 hours with a 4 tier wedding cake and groom's cake. I had everything filled, crumbed & iced and then froze, and individually boxed each tier. I put the boxed cakes in my gigantic coolers with dry ice. After arriving and putting in frig overnight at location, the big layers were still frozen the next morning! Next time I travel for a long distance, I will not bother with dry ice and just travel with frozen cakes in the cooler.
I decorated onsite. My wedding cakes were all butter cream and groom's cake was fondant. I would definitely fondant/decorate onsite.
Or you can follow BlakesCakes instructions - -
Finish and decorate your cake and put each tier in a separate box. Double wrap with plastic wrap and then wrap in foil. Freeze. Travel with the boxes in coolers sitting ON TOP of dry ice. When you arrive, or the day before you need them, put the boxes in the fridge. The day of, set them on the counter and don't unwrap until they are room temperature. This way, any condensation will be on the wrapping and not the cake.
I drove 1400 miles over 23 hours with a fondant wedding cake and a bc graduation cake. After being frozen for a week and the "fun" trip, I wasn't expecting much but they both turned out perfectly.
The guy at the UPS store where I got my dry ice said that dry ice should be stored on top of boxes..because the cool air will travel "down". I overbought on the dry ice and I had it on top of the boxes and all around the sides. Maybe that's why my cakes were still frozen.
I forget how long dry ice lasts...there is some formula..how many pounds lasts per hour. I bought $90 worth of dry ice! Had way too much. The next day most of my dry ice had not evaporated that was in the cooler. The dry ice I had in another plastic container..that did not seal was almost completely evaporated in 6 hours..it was one of those stackable plastic things. With your long trip, you may want to find dry ice locations to buy more. Start with UPS stores.
AI love in charleston. Publix sells dry ice
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