So you're pulling with a pick-up? I've seen some trucks pulling 5th wheels that have a deflector on top. Of course, 5th wheel trailers come up a lot higher. I wonder if you could check your gas mileage without a campershell, get a shell with a 30-day money back guranteee, and then use it and see if your MPG improves. If not, return it. Or maybe you could find a shell to borrow?
For us, the AV and HiLo line up perfectly.
2003 Hi-Lo 2703 Classic
2002 Avalanche 2500