I would NOT carry any fuel inside the cab. That is a very bad idea. I have a Yakima Load Warrior basket, that I put spare gear, tire, etc. in.
I put my gear in the basket, then my 15 gallons extra fuel in the factory basket. I would prefer a roof mount basket myself, over one attached to my hitch. I found this one off craigslist for $100. Retail they are about $250.
Here it is with the spare only, when I went to Moab.
The only reason I mounted the spare, was for the extra clearance. After it was all said and done, I figured it was a waste of time. My dad left his in the stock location, and never had an issue with it.
I envy your trip. I have a few friends down there right now. They have been there since after Thanksgiving.