What is the frame, door or cutout dimensions? I haven't seen actual door replacements so I think you'd have to replace the whole door assembly. On my rig, this is what I would need since it matches the dimensions for my sewer door.
http://www.shoprvparts.com/product.do?no=14310F
This link has the door for a pretty good price but don't forget to compare taxes and shipping. Do an internet search for "ZE102-A".
In my case the item is attached so that the door drops downward. Mine currently does not have a lock and I thought I had seen a door without the lock that matched the same part number from JR Products but this is the one I found. The old part without the lock I saw was a "ZE102" but I don't think that one is available any more.