I've wrestled with this problem myself and have come to the conclusion that you don't need all the relays at all. It's over-thinking the problem (don't feel bad- I did the same thing)
Just wire a charger port to each of the batteries individually, and the batteries themselves can remain wired in series to the car for more voltage. No switches, no relays.
Look here, and scroll down to #7:
http://www.batterytender.com/connecting ... tery-packs
"Figure 7 shows two 12 Volt batteries connected in series. The resulting battery pack voltage is 24 volts. As you can see, each battery is connected to a single 12-volt charger. This is probably the best way to ensure that each battery is completely recharged to its full capacity after each time that the battery pack is discharged. This eliminates most of the problems associated with cycling batteries connected in series strings."
I've also asked an electrical engineer friend and he confirmed that this is indeed not only correct, it's better than one charger of higher voltage powering several batteries.
I haven't done it yet, but I am about to re-rig my daughter's dune racer for 24v (but with two 12v chargers) in exactly this way.
I think one large breaker/fuse for the series battery power, plus small fuses to each charger, is the correct way in this case.