I am in the process of replacing my boat's old batteries with new ones and with replacing and upgrading the battery charging system. I've read quite a bit that's been both posted online on the subject and written in books, and can't help but feel confused. I realize that everyone tends to approach this in their own way, but I would really like the benefit of this forum's opinion on my current path.
First of all, I'm replacing two very tired 8D batteries that not only support the house load but also crank the engine, a Ford Lehman 120. The existing battery charger is probably original to this boat. It is controlled by the original battery switch that I have to turn off and on depending on the charging need. This, by the way, is why the batteries are all but dead. User error.
My biggest power drain is the newly installed Espar Airtronic heater, which refuses to work with the existing batteries. Probably my next big power drains are the electric macerating toilet and the refrigerator.
I'm using the existing battery boxes and installing 4 6V golf cart batteries for the house bank, and a type 31 cranking battery for the engine. I'm replacing the existing battery charger and adding an ACR combiner. Here's what is now installed before any changes (sure hope I understand these image tag):


My challenge is to dismantle the existing charger, install the new switch and wire everything appropriately, using as much of what is already there as I can. My plan is roughly laid out in this diagram.

I bought this charger, switch and ACR unit:



I've based this solution on my own (often imperfect) understanding of what I've read up on all of this. I'm not sure about wire size but think I should probably match the current wire size at the batteries all the way to the switch, the alternator and the ACR unit.
Would anyone be willing to offer opinions on my approach to this?
Thanks!