The way I see it, and this is only my humble opinion, side skirts seem to work because of the side effect of controlling air hitting the rear tire. Look at how your skirt hits the rear tire.
The air under your truck is 95% passing air and 5% cooling air, which is contracting as it loses heat. For this reason as much as any other you have a low pressure area under the truck, and this increases the depth of the low pressure area behind your truck. Great for handling, doesn't really help with reducing the cd behind your truck.
If you have a full belly pan, so air under your truck is not an issue, directing air around the rear wheels is a win, and it would be worth a good ABA test to see if it helps to put thait air outside the truck, or inside, under the bed.
I suspect that if you put that air under the truck instead of outside, it would have a smooth ride under your belly pan for about 30" then help reduce that low pressure area behind your tailgate.
If you look me up, you'll see I am on a similar path you you. I plan to do ABA testing to see what happens with these options... It would be a good heads up if you did the same