2017 montana

the 5th wheel was plugged into a 50 amp box. it was plugged in continuously, it even had a surge protector inline.

Not clear from your wording (honestly due to lack of punctuation), but sounds like it sat for a year without being plugged in. And when you plugged it in your 12 volt stuff was not working. There are some devices that always draw 12 volt power and will drain a battery over time if not connected to shore power. Likely your battery was drained completely.

Did the 12 volt stuff work after you pulled the charger back off? Or only while you had the charger on? You may just have a bad battery that will not hold a charge. I would pull the battery and take it in and have it checked.

The most common causes:
1. The disconnect switch in the convenience center is in the wrong position. The labeling is confusing at best. The key is if you can remove the red key, the battery is disconnected.

2. The batteries were disconnected and/or replaced and wired backwards (even if for just a second or two). This blows the protection fuses in the converter. Replace the four fuses and things are good (assuming the battery is connected correctly)

3. The 50 Amp circuit breaker has failed completely. You can jumper it to verify.

If none of the above apply, get out the voltmeter and follow the voltage from the converter to the battery (or vice versa). Plugged into shore power you should see +/- 13.4 volts all the way down the path.