I built something similar a few years ago. I've attached the diagram and PIC source code. It has worked flawlessly for around 4yrs and we go camping often. It can supply a max current of 15A at 14.2V to the battery with apx 12V input after wiring voltage drops. If you decide to build it, I can supply more details about the coil, etc.
The reason I went this way, rather than use thick cables, was that I wanted to use the standard trailer connection to my camper and not have to unbolt #0 cables every time I wanted the camper off the truck. If you unbolt heavy cables connected to a battery enough times you'll eventually weld them to something grounded like the truck box!