Nice, I have a couple of trucks too but they generally only leave the yard when they are loaded.
Looking at this website I think you might be able to implement some of their research into your design- check out the pictures near the bottom and the chart quantifying their improvements-
Aerodynamic Teardrop Trailer
I found it especially interesting that the hump showed a reduced wake behind the trailer even though in the flat top version the air was re-attached long before it gets to the back already.
I've seen some people here that are absolutely opposed increasing frontal are but this data clearly shows that increasing the frontal area by 12.5% reduced the force of drag at the same speed by 36% so I'd say the shape in more important than the size (at least in this case).