Topper while pulling TT help MPG?

I don't think it makes much difference with or without a topper. The added weight of the topper would cancel out any fuel efficiency gained by little if any improved aerodynamics. There isn't much that can be done to improve the aerodynamics of the front wall of a TT. Even a side wind blowing against the TT will decrease fuel mileage.

I have always had a topper on our pickup, not for aerodynamics, but for covered storage space.