Hi JJPope,
Congratulations on the Sundance and welcome to the Heartland Owners Forum.
This is actually a pretty good question and one that's asked frequently. Your interior 12V lights and other devices should run without a battery if you're plugged into shore power. There's a power converter that converts 120V AC power to 12V DC power to both charge the battery and to supply power to the fuse box. And of course all the interior 12V lights and other things get 12V DC through the fuse box.
If none of the 12V devices are working, and you have a dead battery, it sounds like your power converter is not working. It could be defective or might just have blown fuses, especially if someone was working on the battery. It's also possible that it's not getting 110V, either because the 110V circuit breaker has tripped, or perhaps the converter's plug is loose and has vibrated out of the outlet it's plugged into.
First thing to do is check for a tripped 110V circuit breaker. Sometimes they only move very slightly when they trip, so to be certain, you may have to flip each breaker OFF and back ON.
If that's ok, next you should make sure the converter is plugged in and that the converters internal fuses are ok. Unfortunately, the converter is usually located to the rear of your pass through basement storage and you'll have to take down part or all of the rear wall of the storage area to get to it. There's usually a removable panel on the off-door-side and you may be able to get to it with that panel off.
If it's plugged in, I'd use a volt meter or a drop light to make sure the outlet is hot.
There will typically be 3 automotive spade-type fuses plugged into one end of the converter. You'll want to check those with an ohm meter to make sure they're not blown. If those are ok, you may have a failed converter.
If you're the original owner, you may still be under warranty and your dealer can handle all this for you. Or if you'd prefer to use a mobile service person or an independent shop, you'll need advance approval from Heartland for warranty work at a non-dealer location. If you use a mobile person, you'll have to pay the travel time charges.
The trailer manuals usually are packaged in a large blue pouch that's tucked into a cabinet or drawer. If they're missing from a new trailer, your dealer should get a replacement set for you. You can also download a copy of the trailer manual from the
HeartlandRVs.com website. On the forum, we have a manuals section that has several hundred manufacturer manuals, probably including copies of most of what would have been in your blue pouch.
Here's a link.
Finally, we have some owner-written
user guides and
troubleshooting guides that may be helpful to you. One of them is the
12V Block Diagram and Diagnostic Guide. If you have a volt/ohm meter, it can help you diagnose problems like the one you're experiencing.