I know this is an old thread but I have the same setup as you do a 2023 Mallard M210RB that I towing behind my 2024 GMC Sierra Crew cab with the 5.3L. I get 20plus mpg when just driving (freeway) but I'm lucky to get 10mpg when towing. I just took a small trip to Falling Waters campground using I-10 the whole way. I got near 9mpg going there doing 65mph. Coming back I saw 8mpg or as low as 7.6mpg if I pushed it up to 70mph.
On a trip this summer to Indian Harbour, Nova Scotia from Sunbury, OH I saw gas milage from 7.6 to 9.1 I always drove 65 or less. I was concerned there was an issue but I guess it's just the nature of the beast. I did cut way back on what equipment I was carrying but that didn't seem to improve my gas mileage.