https://ecomodder.com/forum/member-a...l-trailers.jpgI recommend that you visit the website for the Airship 1, telescoping, RV travel trailer, with an eye towards the relationship between the tow vehicle & trailer as a aerodynamic singularity.
A boat tail cannot function until all turbulence upstream has been eliminated.
All that void, between the front face of the sleeper and the tongue needs to be enclosed by some sort of structure which integrates what you have, to the back of the tow vehicle. This includes the trailer's underbody.
The 'lightest' solutions would be to:
1) find a commercial awning manufacturer who can create an aluminum skeleton, and then cover with a taut aerospace fabric, as you see used with inflatable advertising.
2) find an auto/marine/ aircraft upholstery shop which can tailor and sew an inflatable envelope, strapped to the trailer 'floor', which takes on the desired form, when connected to a 12-VDC blower, powered by an electrical umbilical cord passing forwards to your battery.
3) or find an old high school, gear-drive SINGER sewing machine, from a home-economics classroom, and teach yourself how to do it yourself.
I'll try and link a photo of a commercial product from England which was never offered for sale in the USA which will give you an idea of what I'm talking about.