I am making and transpoting a 12 1/2 x 15 1/2 Layer cake from OK to FL, Is this doable

A flat sheet cake?   Yes very doable:)   Freeze it, wrap well in plastic wrap, then foil then put in cake box which is placed in a plastic bag.  Get a heavier cardboard box that’s bigger; freeze several jugs of water and place them between the cake box & the big one then wrap a blanket over that.  Keep away from the sun as much as possible.  

I agree with kakeladi.  A couple of years ago I did a large sheet cake that was transported 23 hours for a party.....in the back of a pickup truck!  It was packed similarly to Lynne's description with dry ice replacing the frozen water bottles.