2021 Tag XL Boondock battery power will not engage

Hi everyone! Our kids are grown and my wife and I finally bought the teardrop camper we've been talking about last Friday, but we're having an issue - the battery power will not engage. We can get shore power and all systems are functioning so we're pretty sure those fuses are OK, but when we engage the battery - nothing. The battery engagement switch is definitely 'on', I've had the battery tested and it has a full charge, and I even swapped out the battery fuses, but it's just not working. I called Nucamp and they said I was doing everything correctly, but before I have to drive it back to the dealership (who we haven't had the greatest purchasing experience with) I was wondering if you kind folks had any ideas? Thanks if advance for your help!