this is a tough one. for a gass engine
(a) ignition advance goes up with higher vacuum, and that offers a significant increase in fuel efficency, even in a FI engine. the more advance you can get, the better the fuel economy
(b) engine frictional losses are related to RPM SQUARED, so higher rpm= higher frictional losses.
So, in your case (a) is definitely working for you with a noticeable increase in vacuum and likely more ignition advance, but (b) increased rpm is working against you as well.
However, I think that the difference between the two when towing is likely to be 1mpg or less
