Battery that Fridge runs on was dead

[Kognyto]

So, my husband found the batter dead and he took it in to the dealership. They fixed it. However, we are wondering if the tankless water heater drained it since it was left on but shore / land power was plugged in. Our main full wall slide would not go out on own either.
...we have a cruiser 35 Searay Sundancer but we are new to Rv'ing.
Any thoughts on why the battery died?

[Jagiven]

Being an used camper. Do you know how old the battery was? I suspect a bad battery, but that does not explain the water heater and slide out issues if you were connected to shore power.

When connected to shore power the converter will keep the battery charged up and will run all your 12 volt devices like your lights (did they work??). The battery in this case is a buffer any high current power draws like moving the slide out.

I'm suspecting a bad convertor, and it is not charging the battery. Maybe the 120V AC circuit breaker is tripped or turned off.

By chance do you have a multimeter? With a multimeter, set it to read DC voltage. Place the leads on the battery. If the converter is charging the battery the meter should read something like 13.7+ volts. If the battery is fully charged, disconnected and had been resting for an hour, it should read 12.6/7 volts. If you are connected to shore power and the battery is reading less than 12.7 volts your issue most likely is with the converter.

[JimD]

The tankless is a propane device that should use minimal power while in the non-heating mode. Batteries in RVs can go dead by just sitting a period of time because there are parasitic draws that eventually suck the power dry.

But, being plugged in should have prevented any issues as it would be supplying power to the battery/12 volt circuits all the time. I would say the problem is in the area of the converter which is what supplies the 12 volts to the rig and charges the battery.

The converter input will be protected by a circuit breaker so that is the second place to check. The first place to check is to verify that you were actually getting 120 volts to the rig. Did any 120 volt item function? You need power in before the converter can do its job.

If no power in, check the outlet you are plugged into first. It will go into the rig and go through the main circuit breaker.

Before you put your "repaired" battery back in service, you need to verify that it will be charged. Also, their repair probably consisted of a simple charge. When batteries get really low they are damaged and will no longer be able to provide extended deep cycle juice. They may test good for voltage but won't have what you need when not connected to shore power.

I have attached a couple of troubleshooting documents that may help.