When I got my Cherokee, I was also confused as to why it only had one 12v outlet in the front (I'm sure every other Jeep in history has more than one...)
I ran a seperate line strait from the battery to a new fuse box, which I located under the plastic flashing in the passanger's footwell. I then ran seperate wires to 3 new plugs, each with their own fuse - A standard cigarette lighter socket in the back seats (for my daughters DVD player) and a cigarette lighter plug & a merit plug in the boot (for matress pumps, mini compressor & fridge etc).
Until I come up with a solution to installing a second battery, I just have to make sure I use my gadgets wisely as to not flatten the battery.
Ben.T