Just consider it a 12 volt Battery.
"Battery" is defined as a group of things,"gun battery " is a group of guns.
A 12 volt starting Battery is a box with 6 cells, the Battery you have is two boxes each with three, same thing as far as the charger is concerned, connect to the 12 volt point, red to the positive end point, the black to the negative end point, no connection in the middle.
Also confirm your converter, most will charge while plugged in, but unless they are the newer 3 stage unit they overcharge the batteries and also pull extra utility power, turn off converter and Battery switch and have Battery minder type device connected directly to Battery.
Best for Battery and your power bill when in storage
Tony & Lori
1989 Country Coach Savannah SE