I have two of the Penguins in the heat pump configuration. One is a 15,000 and I believe the other is a 13,500 but I'm not entirely sure and haven't gone up on the roof and pulled the shroud yet to find out. They are both factory units and from what I understand are still being used by Jackson Center in current production.
For a longer trailer you really do want the 15,000 BTU size.
Lewster, here on the forums, is a Dometic dealer and can help with part numbers and ordering.