Hi sunnyd,
Welcome to the Heartland Owners Forum.
Most of the lights in your coach are powered by 12V DC, separate from the 120V AC that goes to the outlets. If one area stops working, but other lights are ok, the first thing to check is the fuse box. I've attached a picture from a Landmark 365, but your fuse box is probably the same.
With the fuse box cover off, you may see LEDs on the left side of each fuse. If one of them is illuminated, that means the fuse is blown. Note that the LED will only light up if the circuit has a load on it (and the fuse is blown); i.e. the switch for that device is turned ON.
If you don't have the LEDs, you'll have to remove fuses one at a time to inspect for a blown fuse. The best way to do this is with the ohms setting on a Volt Meter. If you don't have a meter, a visual inspection may find the problem.