This always bugs me. DC to ac to DC again.
I wish computer manufacturers made DC to DC power supplies more available (a cord with a voltage limiter in it)
That reminded me of something I forgot... Losses! Google says that inverter is 85% efficient (Peak, which is usually around 50% of rated load).
So, to supply 1000watts, it would draw ~100A @ 12volts. So, if you expect to use that inverter for what it's worth... Minimum 4AWG from the battery to the inverter, and make the run from the batteries as short as possible. If it's longer than a couple feet, I'd opt for 2AWG. Or just roll with it and see if it gets melty. Or maybe the inverter will just beep and cut out all the time from the voltage drop.
Did you use the right pipe dope? Did it have ptfe in it? Also, make sure to put pipe dope on the flare part of the connection. It will act as a gasket.
Pipe dope should be used like Loctite. More is better.