Plenty of guys here are running dual batteries, so you should get a lot of input on different systems.
I currently have the Off Grid Engineering Complete Dual Battery System with a Redarc DC-DC charger that allows me to keep my factory battery and add a aux battery charged up. I've been waiting for some power coating to come back to have it wired up, but it should work nicely with what I'm intending to do with it. The Redarc is also ready for any solar panel plug ins if you decide to go that route as well.