My experience is that wind resistane is a larger fator for fuel usage than is weight. I used to have a 9500 lb 33' tt and got between 7&8 mpg. Moved to an 12,400 lb fiver and milage is no different. I have noticed that wind has a significant factor in mileage. Strong head winds can reduce hwy mileage to 5 or 6 mpg.
Steve Rinker
Rochester, NY
2013 Keystone Sydney 340FBH 5th Wheel, 12,280 lbs loaded (scale)
2015.5 GMC Sierra Denali 3500, SRW, Duramax, CC, Payload 3,700 (sticker- not scaled yet)
Take my posts for what they are, opinions based on my own experiences.