I believe, ethanol fuel, has been and will continue to be, a major headache for those of us, who have "intermittent use" engines around.
While there are "stabilizers" on the market, which cost as much as the gas, I don't think, they are the answer. Of course getting rid of the ethanol, would be the best approach, to this particular problem.
I was wondering, if, when not using the idle equipment, replacing the gas cap with a "non-vented" cap, would help? Possibly, even "lightly pressurizing" the tank, would help. If the ethanol attracts water vapor, the water has to come from outside the tank (through the vent?). Also, if the ethanol evaporates, it needs space to do so. Some believe, that a nearly full tank for storage periods, reduces the problem.
You can get ethanol-free fuel with a little effort, check the puregas.org website and it gives a state-by-state listing of stations that have it available. It also lists octane ratings. Anyplace close to a major body of water will have ethanol-free fuels readily available. You can transport up to 110 gallons without permits, etc. For long term storage consider using an intially higher octane as it degrades with time.