The problem is the truck already breaks up the wind on the leading surface. You will notice Semis don't have V-nose trailers. They work the front of the tractor to be aerodynamic and keep the trailer as close as possible to the tractor cab. Same situation with trains.
There are ideas that could be implemented but even heavy RV users put on so few miles, it's just not worth the trouble.
A solution that would be interesting (but probably not financially viable) would be a fabric that connects from a mount at the back edge of the pickup cab to the front top edge of the trailer. The tricky part is turning where it's not a fixed geometry.