This is the best deal I have found. I got mine installed and it seems to work. I about had a heart attack when I plugged it in and it was putting out 15V. It does that when you first plug it in and within about 15 mins it drops the voltage to like 13.6V then to 13.2 V.
I am using one group 24 battery for now. I don't think a 27 will fit in the battery boxes. I can use two batteries but don't need one for anything accept controlling the voltage and emergency brakes. The size battery and number of them should not matter to the power converter. It will just take longer to charge 2 dead batteries than one. You can get more converter amperage if you want shorter charging times or it you are a 12V