i used a 6 gauge wire for my setup from the battery to the back of my 4runner. here's my portable dual battery setup. i use dc-dc redarc charger, 25Amps. to the op, in your case you need to use a 4 gauge wire. see my video, not too much but enough to get some idea.
https://www.youtube.com/watch?v=ZbVtZvK-f9s