50 amp breaker trips drops voltage

[Cobiaexcel]

I just purchased a 1995 Wellcraft Excel 5.7. The volt meter would start to lose voltage while boating from 13-14 down to 11 on both batteries. I replaced the alternator (marine grade) and both batteries, also marine. I noticed that when running on # 1 battery, the voltage will drop down to 11 or less. I've switched over to # 2 battery, and all is fine boating for the day. I' also noticed that' I've switched the dual switch back and forth a few times, and then # 1 battery will be reading 13, and the engine will crank over. I was thinking I had a loose wire in the dual switch, but just noticed that the left engine mount 50 amp breaker is tripping, and when I reset it on # 1 battery, the voltage jumps right up from 11 to 13. Yesterday, I ran the boat all day on # 2 battery with no issues, however at the end of the day, that same left 50 amp breaker was tripped. What could be causing this? and how come it doesn't affect the# 2 battery operation?? Could it be that dual switch?

I appreciate all your help and advice, chris