I'm sorry I don't have an answer for you. On one of our trips we pulled a small Harbor Freight tag-along trailer but I wasn't monitoring our gas mileage at the time.
These types of questions always puzzle me, figuring most folks are worried about the loss of mpg while towing. Ok, at what mpg loss will you decide to not pull a trailer and just stay at a motel? Or stay home?
I pull an Aliner pop-up camper with my Toyota Prius and folks ask me what it does to my mpg. I tell them I don't give a flying farkle. I want to go camping and the Prius is all I have to tow with.
(It actually goes from about 40 mpg down to the mid-twenties. I run at the speed limits while towing and on some quick hills my right foot may be on the floor. I still probably get better mileage than a pickup or SUV would.)
Go camping, don't worry. If you can't afford the extra gas, camp at home in your back yard!
Charles