Even converters can be effected by low voltage. If voltage is 10% lower your converter will output 10% less power to your battery. Anything with a motor can also be effected. Some motors need a certain amount of power to start, if the power is not there it will continue in it's starting mode and could burn itself out.
Yes your right on the toaster etc, but, it will use more power to do the same amount of work, ie running longer to heat your water, which will make it more likely multiple systems will be running at the same time. Low voltage will effect every system in your rig, some in harmful ways, others in adding to the total load on the system.
2011 Toyota Tundra double cab