Look, I'm no battery expert, but this solution still sounds dangerous.
Here's why I am nervous: Those batteries can still outgas while not charging, AND that fan motor can and does spark. This is why paint booths use special fans that won't spark, to avoid explosions. I urge you to consider moving your batteries to a properly vented enclosure soon.
Originally Posted by matthewsx
Yes, high current stuff should be located close together to avoid having to run large wires which are heavy and expensive. Here's a link to the blue sea circuit wizard that can help you figure out what size conductor you need for your circuit.
It's based on ABYC (American Boat and Yacht Council) standards, but I find there's a lot of crossover between the work I do on boats and our trailers.
One thing I've done though that's definitely not to "standard" is when I put two lead acid batteries in a non-sealed area of my trailer. What I did to make sure hydrogen gas wasn't building up is a put a small fan in the bathroom closet that runs off 110v plugged directly into the outlet my converter is hooked to. That way whenever the charger is running the fan is circulating the air near the batteries. Not a perfect solution but it got me out camping when I didn't have time to build a vented box. Not suggesting anyone else do what I did, just applying a little common sense engineering to solve the problem of a busted battery box and wanting two batteries for the trip....