"I can't seem to find what looks like a battery charger.
Not knowing the year or model of your R25 it is hard to say where the charger is. On our MY '09 the charger is mounted in the battery compartment between the gen set and battery tray. I have seen them (chargers) mounted under the port side passenger seat cabinet all the way to back of the cabinet. I think at one time they were actually mounted on the battery compartment lid underside. Easy way to locate would be to follow charger lead from battery back to the charger. The lead (guessing about a #12 wire without being on boat to confirm), at least on our tug, has a spade type automotive fuse (30amp) about 10-12" back from where it connects to positive side of battery. We have three leads going to charger, one from start battery, one from thruster and one from house bank. Our charger is a ProMariner (black with orange labeling), do not know what was used before or after MY '09.