[dieseldave56]

Gasoline can be stored from a couple of months to a couple of years , depending on storage conditions. The gas you buy at the pump

is a couple of days to a couple of months old from storage.

What makes gas go stale is evaporation, oxidation and contamination.
Usually the lighter , volitile chemicals evaporate first, leaving a less 'peppy' fuel. Less BTU.

The oxidation takes effect as oxygen reacts with hydrocarbons to produce varnish and gum. The fuel will smell sour and turn dark. Not a good idea to use fuel in this condition.

Contamination i.e water condensation, bacteria (from the water) and rust
( from the water as water settles to the bottom of the tank, your steel metal tank).

Ethanol , like California's reformulated fuel is reported by Chevron to store as well as pure gasoline, But ethanol is hydrophilic which attacts moisture from the air so theoretically it should become more easily contaminated than pure gasoline.

As for adding a stabilizer two years later , it is waste of money.

BTW, exercising your genset UNDERLOAD periodically for an hour or two is recommended, but NOT good for your motorhome's engine, as the engine or exhaust will not reach operating temperature . Thus fuel will pass by the rings , down the cylinder walls and into the crankcase, diluting and contaminating the oil. Unless you take the coach out on the road and bring the engine and running gear up to operating temps you are doing more harm than good.