You can sometimes see "voltage" by not be able to push adquate amperage through the cable or wire.

I assume you have the same type of batteries on both banks (lead acid, gell cell or AGM)? If it's a Gel Cell or AGM, you can see if the sides are "poofed-out", that would indicate overheating; or sometimes the vents will "blow" if you outgassed due to a bad cell; or, sometimes it "just dies" for while before it displays something. The lead cell is easier to diagnosis (in part, because they're easier to damage)...one dry cell or measure with a hydrometer each cell...even then you might not see it.

I think the next issue is the connections. You may just have weak connection or bad cable.