I'm trying to decided on an open or enclosed trailer, and it's now down to gas mileage, what are your numbers on Diesel mileage towing a trailer and not ??
I've pulled trailers weekly for 20+ years, never once pondered fuel mileage when buying (or building in my case) a trailer. I get what I want/need and the truck mpg is what it is. A lot of factors go into how a trailer pulls. I have a 24 ft 102in wide enclosed that gets better mileage loaded with 10k lbs than it gets empty. I've pulled a 12x6 5 ft tall enclosed dual axle that was BY FAR the hardest pulling trailer I've ever pulled, 3-4 mpg empty. We kept stopping to check if the brakes were locked up or bearings out. Thing was brand new, just pulled like a boat anchor.
My point is, get what you want/need, not what someone else gets good MPG with. If you're using a diesel to pull a bike trailer, you shouldn't even know it's back there.