Actually there is about a 4 inch hollow wall between you and the fuel tank. the wall provides major support and stiffness for the rear suspension and chassis. Don't remove it.

From the seat area there is the wall, then the fuel tank and then a titanium alloy cover. These are all safety features. If your car is ever in a MAJOR accident, you and the fuel tank are the last to be damaged.

I would suggest going with 18" or 19" rims all the way around and keeping a Mercedes donut spare in the back area with your luggage. If you take out the styro foam inserts under the cargo area liner, you can get more of the spare lower down. There is still the hump in the middle for the exhaust tunnel, so I'm not sure just how well you can recess the donut spare.

All that being said, I've gone on a 4,000 mile round trip to see my parents twice in this car without incident. Just be aware of what kinds of roads you are on and pay attention in construction zones and less than nicely paved ones.

I've also made yearly trips to Las Vegas (over 550 miles each way) in this car two years running and will do it again in October of this year. Only once did I have an issue and it was a simple screw in the tread and caused a slow leak. A tire shop in Las Vegas actually repaired it for FREE... I was dumbfounded!!! When I told them I had not purchased my tires there, they said "We know, we always repair flats for free... " I found by talking to the manager of the store, it's one of their ways of getting customers. People who get flats repaired there tend to return for new tires as well as other services. Go figure.

Good luck with your trip setup!