The 7100 is a "modern" switch-mode converter that does not need a battery to act as its filter like the older model 6300s did.
You can disconnect all batteries and run the rig on shore power and get clean 12v at 13.8v from the 7100.
The two 30a reverse polarity fuses are on its DC fuse panel. If you did that crime, you must replace both fuses even if one is looking good.
There should be a fuse or DC circuit breaker within 18" of the battery pos post. This might have blown for some reason. If it is a DC CB, it might be resettable. Look for a little black button on it to push for reset.