What engine do you have? I've had 2 Chevys one a 2001 4X4 8.1 gasser/allison pulling either a 8000# travel trailer, 9500# 5ver or 14,000#5ver would get an average 7.5 towing and 12 not towing. It was running an air injection intake, throttle body spacer and magnaflow single exhaust.
My current is a 3500 dually 4X2 Duramax/allison and is stock. Towing 14,000# 5ver getting 10.5 to 11.5. Not towing 16-18 city/hwy combined. I wouldn't think you would see a major difference between a tt vs. 5th wheel for mpg.