Are you saying he was hooked up to 50A service when he woke up to the battery alarm going off? You're going to have to find the charger it's probably behind the breaker panel. If you're lucky you can open a cabinet or remove a drawer and see the charger behind it.
You need to know if there is 120v AC on one side of the converter and 13.x + volt on the other side. If you determine that you have charge at the converter then you can start looking for why that charge is not getting to the batteries. Remember you can use your mobile phone to stick into dark holes to take a picture of a component and then pull it out and blow the picture up on the screen to look at it.