The only accessory I know of that will boost fuel mileage enough to pay for itself, is a block of wood placed under the accelerator until it becomes a learned behavior…
All of my TV’s have averaged 9 to 9.5 MPG when towing a full height/width TT… trailer weight (between 7,000 and 10,000 pounds) seemed to make little difference, V6 or V8 made little difference, but the V10 got about 1 to 1.5MPG less towing but, a lot less when not towing…. My combined weight for extended trips runs close to 16,600 and my mileage changes very little from Lake Erie to south Florida… sometime worse on the level roads of Florida, but never a lot…
What seems like a good rule of thumb and ball park number to me is, the vehicles best fuel mileage will be halved with a TT on the back…