Hi Happy-Camping-Couple,
Welcome to the Heartland Owners Forum.
I'm assuming you still have interior lights that are getting power from the Power Converter while you're plugged into shore power.
Near your battery, you will find a row of small 12V DC circuit breakers like the one pictured below. They're connected by a copper buss bar, and are usually covered by a red rubber boot. One of them acts as the connection between the Power Converter and the battery. If it trips, you need to manually reset it. The reset button is teeny-tiny and can be hard to see. Sometimes you have to feel for it.
On the pictures, you'll find that the number of breakers and position of breakers are likely to differ from what you have. These are just examples from other models.