I'm thinking DIY battery box inside an RV below the sink.
Quad T-105, plywood or pine, vented to outside with a 3" or 4" pipe and perhaps a little fan.

Questions:
Any reason not to use wood?
Do the batteries need to be raised off the bottom of the box by using cleats?
Do the batteries need to not touch each other inside the box?
Anything else I'm overlooking?

Thank you

Seems to me that easy access will be a primary issue with your plan. If they are ordinary lead-acid batteries you will need to get to them and open them up to add water, take hydrometer readings, clean the top surfaces, check for corroded connections, etc. Doing all of that without enough elbow room can not only be awkward but downright dangerous if metal tools are being used....

I thought about access, myself. If I take the upper shelf out of the area, I think I'll be able to get to them fairly easily.
I just don't see another place I can put them and keep them somewhat close to the breaker panel.
A guy at NAWS suggested I keep the inverter within ten feet of the batteries, so I figured I'd just keep everything within ten feet of the main panel.

I know this is an old post but some one else new may find it. Hydrogen, the being vented from a battery box is much lighter than air and will collect at the top of a battery box and will flow out threw a relatively small tube upward or angled upward. A fan should not be used because a spark could cause hydrogen to combust or explode