Mudflaps: Small solid rubber ones behind the wheels are good. The stones almost exclusively come off the rear tires. If the center of the rear axle is kicking up stones, you need to stick to roads in better condition or slow down. The "brushes are just silly as any larger stones will go right thru them. Solid rubber may flex but at least will deflect them downward.
V-notch tailgate: We got one because the original got damaged and we found one for $50 on craigslist. Went for several years never having any issue with a standard tailgate. Second time we used the V-notch, the pin hit it because a bump at the last second. Luckily it just flexed and popped out of the hinges without damage. We not put it down when hitching.
Anyone telling you they get 3-5mpg due to the louvered tailgate either has an ulterior motive or doesn't know what they are talking about. Not trying to be mean but it's just absolutely incorrect.