Battery Size

The difference in 4d's versus 8d's are as follows:
4D = 210 amp/hours
8D = 255 amp/hours

Multiply the appropriate number by how many batteries are in your "bank" to give you a numberical reference. Marathon Coach has placed 4D batteries in the rear compartment behind the tag axle of the XL and XL2 Prevost coaches ever since the 45ft models were introduced. Six are simply ALL that will fit in this compartment. It does work well and has been the "norm" for several coach converter for over 10 years.

The other consideration is total number of plates you will have in the overall bank, determined by combining all the batteries. You will have much greater surface area, which equates to greater current potential out of the bank, for surge starting of air conditioners, etc with 6-4D batteries versus 4-8D batteries. That is a real world fact you really don't get from just comparing battery to battery.

Hope that helps.
-ben-