The first few months we were on the road full time, we had a 27 foot conventional trailer that weighed about 8,000 pounds loaded. With our F-250 7.3 liter diesel we averaged about 12 mpg towing at 60-65. The same truck pulling our 14,000 pound 5th wheel gets about 2 mpg less towing at 55-60. I believe that the major reason is that the 5th wheel has more frontal area and thus more wind resistance, rather than the higher weight, Of course the additional weight is more of an issue with braking, one reason we have slowed down some.
Safe Travels...