Regarding the rear of the trailer, especially on a travel trailer, I wonder if you could fabricate a small wing & attach it to the luggage rack, on the roof. Just enough to disturb & deflect the air upwards.
I believe that the open-wheel race cars (Indy, Formula 1, etc. ) use a small lip on the trailing end of the rear wing, for that purpose. I think they call it a "wickerbill".
I wonder where a person could find engineering information about what would work in a situation like this.
U-Haul , years ago, did a lot of 'aero' work for their trailers & trucks. That's how they came up with the rounded corners on their vehicles. Even the corners are rounded.
There is a website (AFNash.com) that had that question asked. Some fellow designed and built a system for the front of his trailer that made it quite a bit more aerodynamic. Did it with tubing & shower wall material. He installed a shell on his Dodge pick-up to help divert the air & the 'system' was built to accommodate the trailer. As I recall, it did improve his mileage.
Joe F. (Buffalo)