A discharged battery will deteriorate more than a fully charged battery. The longer and deeper the discharge from loads or self discharge then the shorter the life.
I fully agree that in this case any damage or shorter life would likely be very minor.
What to do:
1. Keep the battery fully charged in storage with a good float charger.
2. Install a battery disconnect switch on are near the battery post.
3. With 2 periodically fully charge the battery with many hours of utility power is the best. Second best is hours of gen time.
I bring home my rig home once a month to charge the batteries when it's in storage and both battery banks have disconnect switches.