Ive used my half ton quad cab truck with an open behind it a few times. No issues . Even averaged almost 14 mpg like that

I can / will say the enclosed behind it is definitely noticeable difference in how it pulls and fuel mileage change from the open.

I just pulled it 2000 miles home . Plan is use it again later this spring / summer trip back east with my car. I was in the 8 to 10 mpg range most of the trip. Speeds ranged from 65 to 75 mph for most of the way.
Im sure a 3/4 ton diesel would be a better option.
BUT,

I have no long term plans to keep the enclosed past the spring of next year . So I have have zero interest to upgrade my truck for a once a year trip with the trailer behind it.