house battery on a 87 gulf stream

[weatherbymag]

has anybody added a 3rd battery on a '87 gulf stream..

looking to add one and not much room in the transom, 2 oil tanks and 2 batteries already..

it has a bracket, so my transom is solid

 

[cgmiller]

Do you have the access hatch where your fuel filters are below the aft section of the deck aft of the main fuel tank? I cut out my deck and put in an actuall access hatch and there is plenty of room in there on the wooden platform where the oil tanks are set in the non-bracket version of the boat? Aside from that the only other place would be in the cabin under the step??