Hi Gang,
A Zip-Dee for a 27' Airstream measures 16'-11 1/2" according to Zip-Dee (YES! I'm also a dealer/installer and discount them to Forum members at 15% off MSRP)
Buying a used hardware package is a real crap shoot in that you don't know that the arms are the proper bend for your year trailer, you don't know that the roller tube is straight, and most important, you don't know the condition of the roller spring, one of the mst important components of the system.
Plus, it is not a very easy task to rivet the new fabric onto the roller. Other awning companies like Carefree and A & E use an extrusion in the roller tube that allows you to somply slide the new fabric onto the roller, but it has to be riveted on a Zip-Dee.
For the few dollars in savings, you are looking at a lot of unknowns and grief, IMHO.
Let me know if I can be of further help.