Weight will definitely affect your fuel economy. I think the 2% per 100lbs is overstated.
Not an exact comparison but to give you some idea:
My Eagle 10 got a pretty consistent 7.5-8mpg gutted and empty. I added about 6000 lbs including full fresh water tank (200gallons). Fully loaded I got a consistent 7-7.25mpg.
If the 2% statement were correct, my mileage would have been reduced by 120%
I don't have a lot of history on my Bluebird yet but I can tell you that two trips between Granite Falls and Dayton, One empty and the second fully loaded with all of my household stuff, I did not see a significant difference in fuel consumption.