I would suspect that the converter charger is overcharging the SLA battery. Is the SLA battery a good match to the main house battery in capacity, etc ?. It would be best to use two of the same brand/type/capacity batteries connected in parallel with as short interconnect cables as practical given that the converter/charger is not defective and is a modern type with charging characteristic, compatible with the batteries needs. It may be that the chassis alternator is charging at too high a rate for SLA battery while you are driving.
Call Progressive Dynamics and ask for advice from a technician. Have the brand, model number and amp rating of your converter charger and the brand, model number, recommended charge rate and capacity of your failed SLA batteries on hand. Ask about possible chassis alternator overcharging problems. There will be battery supply places in Milwaukee and other larger cities.
