Thanks for the replies. I don't plan to draw much high amperage on the batteries, the highest being the 45A back to the batteries from the charger, and if I read right from the charts I've seen, 4AWG is within spec for that.
I may at some point put in a 3000W inverter right at the battery bank and plug the shore cord into that if I want to run the microwave or something else, I currently use the Yamaha 2000 for that, but it would be nice not to lug it around all the time.
To clarify, I don't plan to run a separate wire feed from the solar controller to the battery bank. It currently runs to the junction block under the fridge, no plans to change that. I will upgrade the wire from the controller to the block, since I'm upgrading to a 30A Rogue controller (I see peak 18A).
If using the frame for the negative is ok, then I'll stick with that, I'll follow Roy's suggestion to tap a bolt near the battery bank and run the negative to that.
