I have factory roof rails on my 2013 Airstream Interstate, built on a 2012 Sprinter 3500 170" WB Extended. I'm not sure about Coachman or Winnebago ERA. I do know that the Airstream Interstates since about 2015 no longer have roof rails. Probably a cost savings since the awnings can be installed without the roof rails.
You can add roof rails to any Sprinter, the holes are there but just plugged. The rails are available from the dealer. But as you noted you will have to remove the interior ceiling panels to complete the installation since they are held in place buy nuts on the inside.
2013 Airstream Interstate Lounge EXT on 2012 Sprinter 3500 170Ext
Formerly: 1973 Dodge B300 DIY pop-top conversion