If you have an iPhone, check for spam messages in the messaging app on the phone. I’ve had messages pop up on my CarPlay screen and later found them in the spam section of messages.
You mentioned that it has a modified sign wave inverter. I was trying to charge my bike battery but the light on the receptacle in the truck, not the battery charger, was flashing. Do you think the truck was having an issue with the charger? (The charger works fine in my shop)