Yes, the dummy load was on a PV cell, and would be wasteful on a storage battery. But the good news is I still think we can make this work. Here is one idea, get a latching relay. Use one controller to turn it on. Set the trip voltage higher on that controller. Use a second controller to turn it off. Set that voltage lower. There is a potential problem with this scheme: the difference in battery voltage from loaded to unloaded might be more than what you originally wanted to set for the high to low voltage.
So here is a secone plan that solves that: what you need is an "incomplete" voltage divider, made of two resisters, but the connection to ground will be through a relay, which is disengaged while the system is "on". At the same time as the battery voltage drops and the system switches "off", the relay is made, completeing the voltage divider, and thus the battery voltage measured at the controller is divided, even though it rises, we can keep it below the level that keep the system off. When the battery voltage rises and the system truns on, the relay will break, and full battery voltage is measured at the system input.
Now the problem with that scheme is all the relays might not switch at the same time. We can come up with a scheme to make sure that happens.