sounds like a real load.... 6-7kw would definitely show a little corresponding voltage sag. The heater in the wallmount (280ah) battery is a few hundred watts at most, so you may have to monitor your distribution panel to find the actual branch circuit that is carrying the load.
I've used the Emporia Vue3 units (<$200 for main + 8 circuits) to discover some surprising loads
I would suspect:
- water heater
- space heater
- any other heaters for driveway, gutters, animal's water
- an EV charger left plugged in that "preconditions" the cabin or battery temp
