7-8 mpg is really what you should expect from a 35ft AND tow at that speed. I have a 30ft '92 Fleetwood with the same engine, I'm not towing and get a little better mileage from an Allison transmission in overdrive (~12 mpg) under absolutely ideal conditions (no side wind, flat-ish road and 55mph.) Of course that's an extra day off my vacation but heck - what's the hurry?
Also when on the road my water tank is only 1/4 to 1/3 full. There's plenty of water where I'm going, no need to add weight and I sure as hell won't take a shower at the roadside turnout.
Oh and did I mention that I'm hypermiling whenever I have the chance.