8 Gauge Wire

As to whether you can successfully charge house batteries from the alternator with heavier wires, I see different results posted in forums. I suspect it has to do with the way the computer is programmed to prevent the engine battery from being overcharged and damaged. In other words, sometimes it works, and sometimes it doesn't.

Try it, and if you find it isn't working out for YOUR particular year and model, look into getting a battery to battery charger, like the marine units sold by this company:

https://www.sterling-power-usa.com/sterlingpowerusadcinputbatterytobatterycharger.aspx