I've never noticed a MPG increase either. Owned one on every truck I've owned as well (5 now), not counting my 2013 Ram 1500 that I owned for about 3 months before it was totaled.
Some studies say it should increase, others say it will actually decrease very slightly, as the carmakers design the aerodynamics with the bed in mind, so "altering" the aerodynamics by adding the cover can affect the airflow negatively.
Regardless, the changes are so small, the savings (or extra cost) in gas rarely adds up to much, if ever. If there are any savings at all, it will most likely never make up the cost of any decent cover. I use it more for making sure my loads are secure in the bed without strapping them down too much.
I was going to drop some coin on a nicer cover this go-around then I normally do, but the Black Appearance Group on 2020s comes with the Mopar tri-cover, which has treated me well so far.