Ethanol-free gas stores longer. It is available also in sealed gallon cans and will keep for a very long time (years).
Also, additives like Seafoam and Stabil will lengthen in-tank fuel life. Gas or diesel.
And remember, water will sit on the bottom of the tank and can be easily removed with a transfer pump.
We do use additives.
I got a little pump this summer to get bad gas out of an old riding mower that’s been sitting. It works great - prime with 2-3 strokes & the gas starts flowing.
My dad insisted on keeping our generator full all the time (largest portable you can get, 17,500 watts, 15 gal tank). Since we weren’t running it enough to use what was in it in a reasonable amount of time, the gas separated & the generator wouldn’t start/run. I found someone to come work on it - he drained the tank, changed spark plugs, etc. All was well until ...
This summer when Ian hit FL, dad was afraid we would get some remnants, so he filled the tank. No Ian remnants & here the generator sits, full again. So, after yesterday, not so full.
In general, we fill 5 gallon cans to keep on hand if we need gas for the generator, but after about a month, we recycle by putting it in a vehicle, then getting fresh gas (& we put additive in the cans).
Where do you get ethanol-free gas in sealed cans? I know you can get ethanol-free gas at marinas, but we don’t have any within at least an hour’s drive.