You did not state how old the batteries were, but your camping habits caused the batteries to sulfate and lose capacity over time.
In order to have batteries last long they need to be quickly recharged such as running the generator each day long enough to get to 90% charge, and getting fully charged on shore-power right after getting to a powered location.
People who just use battery for a single light, water-pump, refrigerator of propane, have light usage and can charge right after getting home for a weekend.
You have the classic case of finally noticing the reduced battery capacity after adding something new to run off the inverter.
Take better care of the next pair and they will last longer. ;)