Paul, I have a 2002 17'SD and it came with the 24 size battery and
I took it out and put it in my boat, than put a 29 battery in and it has twice the power of the 24. It is a little bigger but it fits with a little mod change, had to noch a little out of the top inside trim so the battery post would go in and had to move the hold down bracket out and re<img src=http://www.fiberglassrv.com/board/uploads/3e8fab94d1d6bDSC00109.JPG/> drill new holes to mount it. It was easy and it is ease to get out to service it.
Here is the Interstate battery site at the left side click on marine / RV
and check out the battery info. http://www.ibsa.com/estore/default.asp
Here is a picture of my battery in the box.