It depends on how different the two batteries in the deep cycle spots are now. What shape is the "middle" battery in? It was paired with the dead driver's side battery. You could be killing off your new driver's side battery.(Is it even a deep cycle?)
You could be giving your new converter an impossible task. It could be (don't know--just a possible) the 9100 died from trying to work with whatever was in the two deep cycle spots; not that the bad converter ruined those batteries (or at least one of them).
IMO the battery bank situation whatever it is, should not be treated as something to sort out later someday.
If the new driver's side battery is a starting battery, you could put that on the passenger side and toss the one in there, which is likely not very new, and then get two proper matching deep cycle batts, such as a pair of 6s, or whatever you choose for that job.
Yes, if you don't use an inverter much, you would not want to buy an expensive inverter/charger, especially since you already have a new converter.