Tundra MPG Expectations

My signature has not been updated but I'm a proud owner of an 08 dc tundra 4x4 5.7. The kid who owned it did a bunch of mods. duel exhaust, k&N air intake (lets the engine breath), updated ecm for better shifting with the mods, bigger brakes, tires, shocks, extra springs. took my keystone springdale 296bh "big White box" approx 8500lbs loaded, up to the white mountains and the Adirondacks. I donot remember the exact mpg but it was right at 8mpg avg speed about 60-65 on rt 16 in nh and in the Adirondacks very little highway mostly 2 lane rds and lots of hills. The chevy k3500 in my signature got 6 or 7 mpg can't remember exactly it was 2yrs ago the tundra was this year. So to get 15mpg it must be absolutely perfect conditions no hills maybe a long down hill lol..