Hey Sharp018, i have a similar but basic setup to what you are looking for.
I run a third batter in a battery box with a couple plugs and volt meter built in to the case which i generally use when camping and sit it in the tub. Runs from the second battery with just a 50amp fuse I believe. Ive got the end connection as an anderson plug which is handy so i can connect it to a third battery, solar panels or a couple times I have connected 2 extra(125ah) batteries (totaling 4).
I was told the more batteries i hook up to the 20amp redarc charger the slower it would charge, but i can live with that. If i take have 4 batteries (total) I generally take the solar panels to charge the 2 (125ah) in portable battery boxes. If i just use 3 i let the car do its thing and leave the solar panels at home.