What type of quick disconnects for 18ga to 12ga low voltage wire connections?
I did a search for waterproof wire connectors. Get one that has 2 wires and is a screw-type connector with a good rubber or silicone seal. And make sure the connector fits your size wire. 

When in doubt, if there is space where the wire enters the connector, seal with 100% silicone. 

Source: I have solar panels that power a water pump outside and I use these light duty connectors which work fine. But I also put the connectors on a brick, off the ground, so they won't get submerged during a heavy rain. 



But these connectors were tricky to put wire into and assemble. I'd use different connectors next time.

You want IP65 or higher rated connectors, IP68 preferred.