The 2000 watt inverter, just like high quality 2000 generators, is not enough capacity to handle the big spike in amperage draw when A/C compressor calls for power without something like a soft start kit installed on the A/C unit. Look up Micro Air, we have one and it is one of the best upgrades we have done and we run the A/C off our Yamaha 2400 inverter gen.
Next culprit I think would be the batteries. You have a lot of capacity, but you are asking a lot of them as well. Others have covered it, more batteries or switch to lithium which suffer less voltage sag under load.
EDIT: I just reread OP and realized it said 18', as in 18 feet, of 2/0 cable. That is fat cable, but 18 feet is a long length that I suspect contributes to the low voltage fault at the inverter. Put the inverter as close as possible to the batteries and use as short of wire length as possible.
there is no cable listed that is good for a 36 foot return run that can handle a 2000 watt inverter. I missed that in the initial post
to the orininal op, is there any way to move your inverter right close to your batteries then run the 120V output the longer distance? a 4/0 cable used for that distance will give you a 5% voltage loss and you are much smaller than that so I am assuming over 10% loss which means your 12.6V quicly drops to 11.5ish ands then the surge from the batteries will let it drop under the inverter low voltage alarm level. sorry I didn't pick this up at the get go but I like Ewarnusa assumed 18" not feet as no one mounts an inverter that far from the batteries, it even says in the inverter manual to mount as close as possible to the batteries. captdaveo
