If Elliott's Marathon is like mine, and I will guess it is, the 270 amp alternator is 24 volt. This alternator only charges the house batterys. Marathon splits the house batterys up into 2 groups of 3 using 4D size batterys. The invertors are 24 volt and draw from there set of batterys. Just a little more info to help the brain trust! If this is of any help at all!