The largest 12 volt wire will be the main battery connection. Probably #6 wire black-positive white-negative. Negative probably through the floor to the frame. Positive snakes up to the battery.
There might even be space in the lugs or a free terminal to connect the inverter direct. If you use smaller wire than the main feed you will need to add a fuse.
Transfer switch $50 Just plug this into the small inverter. Remove the branch circuit romex and connect to the output. New piece of romex to connect from the panel to the input side. Need to place the inverter where you can turn it on/off or figure out a remote.
