You will need to pull new 12v wire to a 3000 watt inverter. Need 4/0 minimum (not #4) and 4 to 8 batteries to properly drive the thing.
Transfer switch is just a large junction box with two power cables going in and one coming out. Do this right and get help if needed. 120v can be lethal not just risk of damaging some equipment. Some of these just had you plug the main cord into the generator, no transfer switch. In which case you can plug the main cord into the inverter. If you do that you also need to set the fridge and water to propane only and disable the converter.
I recommend sine wave inverter only. Especially if running microwave, computer equipment, or a motor such as a fan.
