The running boards probably hold you back as well. If you can get in and out without them, take em off! I think it's safe to say none of that RV stuff was wind tunnel tested, so it's probably all taking a toll.
Maybe you can use some clamps or stock bolt holes inside the rear wheel housings to add skirts to them. Make sure you can take them off easily if you need to change a tire.
Cliffs Notes on the air dam would be, as others said, to drop it as low as the lowest component. You may be able to score some really tall lawn edging, but otherwise black coroplast is good. To shape it in plan view, here are some quick ideas-
lawn edging: either attach aluminum bar behind it and create brackets to mount it to the car, or cut a wide wooden board to the shape you like and drill the edging to it.
coroplast: either use the aluminum bar method or buy some long steel rods from a hardware store and slide them into the plastic's channels. This way, when you bend the piece it will stay bent. I've yet to see anyone use this method for an air dam, so you might want to put a lot of rods in to start.
Of course "Preparation H" (When Preparations A through G have failed) would be to find and modify an aftermarket or special bumper or air dam designed to fit your vehicle.
This would be a good start.