Been using a louvered tailgate for the last 15 years, on 4 different trucks, 2 chevys, one ford, one dodge. Do they help mileage? Have no idea. My sole purpose was the v cut in the gate. On my setup when the pin comes off the hitch slightly, it slides right thru the gate with perfect clearance. So for me its solely to make the job easier. If it was cost effective converting my stock gate with the v cut out of it I'd probably go that route.
I also run a wind deflector. Bought a Ultra Fab when we bought our first 5'er. Still use the same one. Does it help my mileage? Yes. but in reality it probably took all those years to get the cost back. Couple years back I did my own comprehensive testing. Camping about 100 miles from home. On the way up I had the fiberglass wing off and stuffed in the trailer. Reinstalled it for the ride home. Wind was pretty much a non issue, light to no traffic on the interstate both ways and fairly flat. I got 10.5 without and 11.5 with. So yes it works, and yes it takes forever to recoup your costs in mpg savings. BUT! what I also discovered (besides the bug issue), is the truck actually felt like it wasn't working as hard with the wing as it was without it. So in my mind, it saves gas, keeps most the bugs off the front, and saves wear and tear on the truck.
As another cost saving measure, I've had 2 birds so far go over the cab and smack head on into the wing over the years. What the cost of repairs might have been if said birds went full tilt into the (a lot thinner) front cap is anyone's guess, but I imagine it would be more than the cost of the deflector.