Here we go again yet another Trombetta "Big Boy" failure!

You can get one battery down more than another if they are not balanced for load/charging. Be sure your load/charging wires go one to each pair in the series/parallel pair. Try not to have a "downstream" pair of 6s that does less of the work.

It can be difficult to do that if the rig's wires are too short so they won't reach the back pair in a battery tray. You can extend those wires to make it work