Gas mileage, environmentally friendly remodel

Nope, more like a displacement hull. OTHER laws of physics dictate basic MPG. In the case of an RV, on flat ground the OVERWHELMING amount of HP is used to overcome AERODYNAMIC DRAG. Yes, on climbing grades, weights becomes the dominant factor.

So, if you live and drive in the mountains, lighter will give you better MPG. If most of your driving does not involve serious climbing, it will make little difference.