You should be using a larger wire. In your math you are using 1500 watts and diving by 12 volt. The 1500 watt rating of the inverter is the AC side. That would only work if the inverter was 100% efficient. Most are only around 90% efficient. The majority of the 1500 pure sine inverters I have seen are rated for 150 amp on the dc input side.
Which would be 1/0 wire. I would check your inverter to see what its amp rating is.

I use the EZ lube axles just like Ray said, Just use a hand grease gun and not a battery/air gun. If you even have half a brain and have just some mechanical ability you will be fine
I grease them with a gun every year and every third or fourth year pull them apart the check the bearing breaks and seals. I have been doing it this way for decades and never had a single axle problem.