It might not be bad at all. You have to check its voltage up where it is, not down by the battery which might have an "open" in the path between it and the converter.(EDIT!! Nope, your lights got brighter on battery when charger on, so the path is ok) Or you could have accidentally caused a reverse polarity for a moment while installing your new battery so the converter just needs a couple of fuses.
Or maybe the converter is not getting any 120v input even though you are plugged in. What's this about the AC also not working? (The plot thickens!)