It's more likely the cables are the problem.
When the water pump is activated, the voltage drops at the inverter because of resistance. We got battery resistance and cable resistance. A good battery may have 12 mohm resistance. Cables can be 25 mohm or worse. In this case, the cable will drop twice as much voltage as the battery.
If you got 2 12V batteries each having 12 mohm, then the combined resistance is 6 mohm. That's a good reason to use 2 batteries. But it won't help much if the cable is too small.
mkirsch wrote:
The interior lights visibly dim when the pump goes into its fast on-off cycle mode so it is definitely drawing down the voltage.
The only cure for that is additional battery capacity. I chose to work around it because it's only a temporary condition that persists for a whopping 5 minutes a day.
