Having a rack over the spare tire is a great option.
I picked up a waterproof bag, that fits into the rack on the LoD rear bumper.
http://rightlinegear.com/4x4-duffle-bag.html
The rack is about 15.5"x 35" and the bag is about 13" tall, so not a huge aero drag on the freeway.
That bag will swallow a lot of gear.
If I was traveling with more than two people and two dogs, a trailer would be high on the priority list.
My friend your always coming up with cool ideas. This bag seems perfect for the LOD rack, nice find! Any photos of this bag loaded in the rack?