From what I understand this is a common problem with most if not all HD guages. I know it is with my 2013 Ultra. By the way, it is wet inside the guage,(condensation), hence the fog. Apparently Harley doesn't feel the need to buy or make guages that are sealed properly.
My gas gauge fogged up on my softail, so I just parked it where the sun would beat on it all day (had to keep moving the bike) Didn't have any fog after that.
You could try blowing dry compressed air, in/around gauges...
Removing moisture from the inside of the gauge is what would keep them from fogging/condensing inside. Might become a daily routine, though, if you actually manage to get the dry air into the gauge housing...