I have a 2006 41 AC. The PO installed a roughly 35 gallon tank under the saloon on the port side of the boat. I was told it was 50 gallon but that was a lie. You just can't trust Canadians or Brokers though I don't know which one was the fibber.
There is a fuel pump to send diesel from the new tank to the original tank under the aft berth. The fuel pump is located in the same area of the bilge where the engine through hulls are located. The boat can go for over eight hundred miles on the engine alone if the RPM is kept reasonably cruising speed. His installation did kill the ability to use the saloon table as a berth until I change the supports out. The tank interfered with the OEM table leg being able to go below sole level on the Port most leg. I have since converted them to pneumatic legs that need no room below the sole. Hope this helps. I have not taken up the sole so I really don't know how good the install was but judging from other projects he did, I'm sure I'll be fixing it at some point.