I believe you can do it! In fact, I have wanted to buy a RAM SRT10 and do the same thing; get 20mpg highway with it.
Aerodynamics will go a long way. I didn't start seeing what I considered "good" mpg numbers until I covered nearly the entire belly. Including the gaps between the frame rails and the body, from tire to tire. My truck has almost everything covered to the rear axle, and I even have panels covering from the rear tires to the rear bumper, save the spare tire area.
Also, I swapped my clutch fan for an E-fan, and so I noticed I needed a type of vent between the belly panels. So, I have a 1 inch spacer where the panels meet, with the leading panel sticking lower and slightly overlapping the trailing panel...in a type of cowl flap effect. Did this for the panels running between the frame rails. I am using 0.040 or 0.050 inch Aluminum 3032, I think.
My engine bay belly panel is ABS, with cutouts for the wheel wells. I do cover the lower control arms also, with part of that panel. The flexing is slowly wearing on it though, I can see cracks developing. But that has taken a couple years.
I use corroplast for side skirts. Maybe I should post some pix in my garage. Double up corroplast if you use it like this; single thickness flops around way too much.
I have quarter circle shaped front fairings forward of the front tires. They look a little like I cut a missile silhouette in half lengthwise. They work great on the highway, but I've torn them up on parking blocks and steep driveways. ABS isn't the best material for that. Corroplast worked better, but looks a bit hideous. [edit] the next version may be corroplast covered with some thin bailer belt.
I tried a grill block, but it was costing me a little mpg (5%). Didn't seem to help my warmup cycle that much, maybe because I use a block heater in winter.
Sorry about the book, but I've been working on this a long time.
