The information you are looking for doesn’t exist, plain and simple.
Towing a 4x8 trailer with a motorcycle loaded on it is going to result in different fuel economy if you were towing a 6x12 enclosed trailer. Completely different wind drag characteristics, and vehicle length.
The best you can do is search out RV websites, and see if anyone has towed trailers similar to the one you want to tow, and what vehicles they used, and what their differences were between towing mpg and non-towing mpg.
I tow all sorts of different trailers with my cars, from flat beds, to motorcycle trailers, to enclosed rigs, and each trailer is different in the way it effects fuel mileage, so I know this from personal experience. Different motorcycles on the same trailer and car combo change the fuel mileage.
BC.