2001 Fleetwood Southwind, 35 R(35 feet), F53 chassis, V10, weight 20500 #, normally drive 60-65, getting about 7mpg.
Previous was a 28 foot 1999 Coachmen Mirada, P-32 chassis, 454, weight usually about 15000 to 15500# (Max was 16500#) ... (ran out of room before running out of weight). Kept detailed numbers and averaged 9.6 over maybe 30,000 miles or more. Both are gas engines.
If we're talking about carrying say 100 gallons of water, about 800 pounds, I don't feel it makes much difference and the Mirada actually rode better with a full tank. I'd rather have the water. As SeilerBird said it's more a function of that high drag square front end and speed.
Howard