Can a DC-DC converter destroy a SLA battery or wear out very quickly?
The switching nature of a boost regulator is typically not a problem for most batteries. The concern here is the current pulled. If it is in the acceptable range of your battery. Or if you let the battery drain too much.  Trying to pull AMPS from a dead battery will hurt it. Even deep discharge batteries would be affected by this. 

But also,  those batteries are very old and you are mismatching them. If you have them in parallel, one will attempt to charge from the other due to their different conditions. So replace both with new and identical ones. 

Additional concerns, continuous load current of a battery is not the peak, surge or pulse current. The Datasheet for your battery will have both, as well as the expected capacity at any given load. A car battery may have a 600 CCA rating, but this is over seconds and not minutes let alone hours.