1800mx battery

[TBarCYa]

I'm still waiting until I can take delivery of my 1800mx and I'm planning my third upgrade, a second battery and switch. What size/type battery is in there from the factory and is there room for another of the same or a group 27 battery in the same compartment as the stock battery? I have a deep cycle group 24 and a deep cycle group 27 one of which gets traded in with my old boat so I need to know which to keep for the new boat. Of course, if the group 27 will fit, then I'll keep that regardless of what the starting battery is.

Thanks.