Show me an instance where a larger engine gets better mpg for a road going vehicle.
Larger engines create excess power which is a waste in every regime except acceleration and ascent. Longer strokes, bigger bores, and large cam profiles all waste energy in order to gain power outside of the cruise regime, but average mpg is dominated by cruise.
When cruise mpg is the goal, minimizing ci is the lowest hanging fruit.
OP says he wants to tow - but does tow mpg matter as much as unladen? Without those details it's impossible to figure out which end of the displacement spectrum to be on. Knowing a max grade, weight, and operating elevation would make figuring out the minimum requirements a lot easier.
Also, typically a smaller engine with a turbo will out power and out mpg the larger na engine in almost every case. It can maximize torque when needed and use fuel sparingly when it's not. It's no secret why oems have had a love affair with boost in ecino cars for some decades.
