It may be able to get close MPG empty because peak power isn't needed, but a gas engine will never get better towing MPG than a diesel. Gasoline simply lacks the energy density of diesel and requires more gas to get similar output.
And that’s fine. I tow once a month or so. If the diesel gets 20-30% better mpg towing that one time a month it had little effect on my overall fuel usage. Diesel also cost $0.30 more on average nationally, and is more like $0.50 where I live so that cost savings is really ~10%.
Here in Utah the diesel twins are popular. I see them everywhere. My podunk local Chevy dealer has like 4 cars on the lot and 4 of them a Duramax ZR2’s. You know how many of them I see towing and not just driving around town? Very few.
I understand that the MPG readout on the dash is really impressive with the diesel, but that’s not the whole story.