Here's what I'd check. On most converters, there is reverse polarity protection fuses. I believe on this model, you need to slide the converter out to check them. It's quite possible that during installation, they may have accidently connected the battery terminals backwards. If they did this for even a second, it will pop the reverse polarity fuses. These fuses are not on the main fuse panel, but attached to the converter itself.
First, I'd check the battery terminals to make sure the tech didn't put the wires on backwards and leave them that way. Once that checks out, then pull the converter out of the case and check the reverse polarity fuses with a meter.