When you say power converter I am assuming you mean a 12V DC to 120V AC power inverter. To do it right, you need to first get a higher amp alternator. The stock alt will not adequately charge 2 batteries. I'm not sure what you mean by "super expensive dual battery charging kit" but yes, you should use some sort of battery isolator. There is no reason to use a high power relay to disconnect power to the inverter as it shouldn't draw any power when it is off, same for the winch.
I currently have a Denso 160 amp alternator is that not big enough?
