Hydrogen is orderless.
Hydrogen sulfate smells like rotten eggs and is created when a battery is overcharged.
What readings are you getting on your voltmeter?
I have Interstate on the 21 Mach 1, 71 GTO and 69 Corvette. On the two older cars I replace the battery at 8 years to be safe. I keep all 3 cars on a battery maintainer whenever they are not being used.
Have you tried locking the car and placing the key fobs a significant distance away from the car?
My 21 Mach 1 battery lasted 4 years replaced with Interstate and reset battery monitor at time of install.
When not used for a period of more than a few days use smart trickle charger from Deltran.