Some good options mentioned already. I would do an aerocap. You could make it with struts on each corner so during normal operation it has the boat tail effect, but if you need to haul you can raise it to a normal camper shell height.
I would also look at a belly pan. I think for trucks,especially if you haul, belly pans are better than air dams (I'm not advocating against air dams - do both.
). Depending on how the underside looks, you might be able to slope the pan upwards toward the back for that tapered effect. You can also transfer that into a rear diffuser.
This is just a half joke, take it with a grain of salt. You could put a turbo, so in v4 mode it will have more power and stay out of v8 mode. Don't know if it would use less gas than v8 n/a mode, but it's something you might want to look into. Don't take anybody's advice here on the forum regarding this though (including me), go to a tune shop so it's done right, if at all.