If you can afford it, place the extra concrete for a full slab. It is a better improvement and makes it easier to work underneath. You may find other uses for a complete slab (like pulling the TT out and using that slab for a backyard party, work on your other vehicles, etc).
I tried to estimate the added concrete for the full slab and it should be an additional 1 1/2 -> 2 yards of mud. That makes it a no-brainer, IMO.