Help understanding RV/Marine battery chargers
When there’s a load, the charger current will be split between the load and the battery. The charger can still monitor voltage at that time. 

When the load is removed the charger can revert to the charging profile, terminating to float voltage at the end.

Reapply load and the process starts all over, depending on how much the battery gets discharged. So a refrigerator cycling on and off would make the charger keep switching modes, based on sensed voltage and its calculation of battery state-of-charge.

The charger can (well, could) also monitor current in and out of the battery to help figure out battery state of charge - that is, ‘count coulombs’ as some Li-ion chargers do. But for lead-acid, mainly the voltage method is used. More here: https://batteryuniversity.com/learn/article/how_to_measure_state_of_charge