I have two deep cycle batteries in my Airstream, in the panel there is a fused bus bar (that appears to be faulty) so the solar panels only charge the one battery. Can I just run a heavy gauge wire between the two positive and two negative terminals? Or do I have to put a fuse across them? ... if so do I fuse the positive or fuse the negative? What's the worst that can happen if I don’t fuse them?
The worst case scenario is one of the cells is shorted and before it explodes/catches fire itself it also shorts the other cell.
The fuses are there for a reason (and there is a reason the fuse went open).
Sure, you "can" bypass the fuse with heavy gauge wire, but you will take the risk.
It is very common to have two or more lead-acid batteries in parallel, with no fuses between the batteries - but you MUST have a fuse close to the batteries, between them and other wiring in the boat/vehicle. For marine use, ABYC says the fuse must be within 7 inches of the battery.
I have had a couple of cases where one cell in one of the paralleled batteries shorted - there was no fire,explosion or other excitement - I just couldn't charge the battery bank to its proper voltage.