The tanks are inside, but yes, there is some crappy fuel about. Suppliers move fuel from tanks and mix it and cross supply each other so it does the rounds. Once you buy some, the crap sticks! Our humidity is often in the high 80s and summer temps are in the high teens. The day tank is generally full so no worries there, but the other storage is not kept full and is a worry as regards condensation.
As a side note, I've had some major fuel contamination issues over the years and HAVE moved suppliers. Not sure the fuel is any better. I've paid big bucks to have the 10,000L tank opened, pumped out, steamed and refilled. I'd prefer not to go there with the genny!