How do you store your 20 gallons? I'm using plastic Blitz cans, stored in a corner of my garage away from the door to the house and away from the sump pump. My hot water heater isn't in the garage.
In an Outbuilding (15x20 shed, in four 5 gallon Blitz cans (Plastic). All are stabilized. Every 2 weeks, my iphone beeps at me and reminds me to rotate the oldest gas. One 5 gallon can is dumped into my car and re-filled at the gas station, and put to the back of the row. The oldest gas is always up at the front of the line.
I store the generator with no fuel in it, and when I do the final shutdown once the power comes back, I run it dry so there's minimal residual fuel. I also drain the fuel lines. This was a lesson learned from the service guys who told me it's absolutely necessary due to ethanol damage, and no additive is better than no ethanol sotred in-system.