Some will say it is The cam cover design on the newer bikes. Some will say it is the gasket itself. Twice now we have had gaskets last over 20,000 miles on a 2015 Rocket X. I hate leaks and that saying a lot since i own a 2010 Fatboy.
You have to get everything out of the way and yes this includes throttle bodies. Some will argue that point. Then you have to clean the mating surface's. Some will argue that. Use silicon only on the four slots. Some will argue that. Then replace the bolt seals. Some will argue that. Then tighten down in 2 to 3 NM using the sequence in the manual. Takes some time but worth it. Tighten to torque prescribed by the manual. Older bikes different torque. Either way you can not argue with my success.