Two banks of four each sounds about max for a single
bank. My last setup of 3 banks of 2 trojans each worked well for about 7 years. When it finally died I changed it to a starting bettery and one bank of 2 AGM's, but in concept
yours should work very well and supply a lot of power.
I'm thinking it may not be the 'batteries' that failed but maybe the charging
sytem that was improper or maybe some other probelm that caused the batteries to have weakened so quickly. Failure to fully recharge or to deeply discharge the batteries very low and often would shorten the lifespan greatly. IT's amazing how you can make the batteries die when you discharge them to 10% capcity on a regualr cycle.
On bad cell could cascade to the rest of the bank but not both banks. Something is probably wrong with the system as a whole and not just because the batteries were defective. Testing the batteries would perhaps tell you what killed them but maybe not why.
I just replaced a starting battery caused by a combination of factors wrong with the system including a bad isolator that was brand new and a miss wired bilge pump
Testing the whole system after you replace the batteries would really make you sleep better. Adding a battery monitor
was the best thing I ever did. I found the last problems before I lost
2 group 4 AGM's (at $200 each) though I did lose a top of the line starting battery (it was not monitored). Testing the shore charger
and all grounds is a good thing to do when your battery banks have problems or you install new batteries. Check the charging
voltages. They most always can be adjusted to some small degree that matters in a big way. Over and under charging are equally very bad. Water
in the cells is only one indicator and of course something to stay on top of too.