That’s good mileage for that size trailer, but everything gets good mileage driving that slow.
I get 7-8mpg towing the toyhauler with the brodozer and 7-8 towing a 6klb boat with a new V8 half ton….at 75-80mph.
Drop that to 55-60mph and I’m looking at 11-12 mpg pulling a camper that is double the weight, way taller and with a truck that is the epitome of poor setup for fuel mileage. 7.5’ tall, and under-geared on 37s with a 90hp tune.
It’s all relative, but love folks who like to feel good about their fuel mileage!