Ya, anything that will reduce frontal area will help... so if you have any way of reducing the height, you will reduce the amount of fuel needed to move your brick through the air.
I've seen some trailer campers that have an interesting popup concept... basically two hard shells which fit inside each other (kind of like plastic cups that fit together), and once you arrive at your spot you can crank the outer hard shell up to the point where you have standing room inside. The following pic is this same concept applied to a Sprinter camper:
Probably a bit complicated mechanically for a DIY project, but a cool idea for reducing the frontal area while driving while still having the space and the insulation for camping. You said you need at least 6 feet on the height of your cargo; so not sure if this could work for you.