I'm with the others here. Most likely your issue is cables not big enough (thickness) and/or too long.
Inadequate cable is the main issue with getting inverters to deliver power. You need really ridiculously huge ones.
I'd add heavier ones in addition to keeping your existing cable to. More is better here and there's no such thing as too much.
We're talking 00-gauge, REALLY heavy ones.
If you can get your inverter within a couple feet of the batteries (which is a very good idea) you can get away with 2-gauge, if you triple them up.