They make longer electric fuel/water sensors like this. Maybe you could drop one right in the opening, to switch on a solenoid valve or something once full.
Maybe you could put multiple overflow tubes going into each consecutive jug, so after one fills it starts filling the next one, then the next one.. Then when it gets to the last one a sensor could close off the solenoid valve.
You're probably wanting a mechanical float though right? I bet you could rig something up, where a long piece of PVC pipe with a cap on the ends could be dropped down in, and attached to a normal float that wouldn't otherwise fit through the filler necks of the jugs....