If you in fact have only one filter, it
MUST be 2 micron! If you have two, the secondary filter closest to the engine must be 2 micron and the primary filter/water separator can be either 10 or 30 microns--either is OK.
If you have a secondary filter it will be installed on the engine as can be seen in this picture
(unless it's been relocated by a previous owner, as I did mine:
Relocated Fuel Filter & Added Priming Pump).
I would expect a 2005 coach to have 2 filters, but it's possible that the chassis was built early enough that it only has one, as there were a couple of years that they were built that way. You should be able to tell by inspection how many you have, and Freightliner certainly ought to be able to tell you given the last 6 of the VIN.
As shipped from Freightliner the primary filter/water separator would have been an Alliance brand (Freightline's brand) and the secondary, if present, would have been a CAT filter.