My post #12 above makes reference to the CTEK Multi US 3300 charger that I use. The jpg with the CTEK is just resting on the bumper is for information - the hood stays closed when I charge and the CTEK remains on a shelf. The electrical plugs are the GM Delphi Weather Pack type and the one resting on the switch box is to plug into the switch box pigtail when not in use to keep pigtail connector internals clean.
Below are jpg's of the switching hookup off the two batteries. The switch box is just that, a Double Pole, Double Throw, Centre Off toggle switch for transferring charging from one battery to the other or neither.
I switch both the ground and the positive - probably not necessary re the ground, but ....
The pigtail tucks up inside the grill when not in use and I can toggle the switch with my fingers thru the grill, hence the hood never opens in normal use of the maintainer.
I have ten amp fuses in the leads between the positive post of each battery and the toggle switch. The CTEK is good for a maximum of three amps - the fuse rating is just to protect from some sort of short.
I do the maintenance charge thing for about a month of nights twice a year, fall and spring. I have a garage that I park the 3 in.
My 3 is a daily driver - gets maybe 50 miles a day perhaps three times a week and 30 miles a day the rest. In spite of that, the charge system never really fully charges the batteries, hence the advisability of a routine top up at least twice a year.
My Interstate batteries last about six years - always die off warranty.