I have pulled TT's and now on my second 5er. There is no doubt that the 5er's are easier to pull. You have 6 to 7 feet more behind your truck now than you had. Your air flow coming off the truck bed is hitting a box now. That has got to effect your millage and rpms.
I read a good article about the mesh tailgates some replace their tailgates with. Because of the air flow gas millage actually went down. Not what you would think but that was the case. Your air flow with a 5er is much smoother. You will also have more resistance from the sides because of the added length with your TT.
I am not saying not to check everything closely just that it does not really surprise me.
07 Dodge 1500 crew cab with 20" wheels, 08 Forest River Rockwood Signature Ultra-lite 8280SS
"The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man."
George Bernard Shaw