Wow, if it's a Two Step, that's a very nice awning! Unfortunately it won't work on most Sunlines because the awnings mount in front of a window on either the front or back, and it needs a solid surface. A Two Step would definitely have a gas strut.
Google doesn't have much help for that part. You've got two options:
1) Go to your local RV dealer and ask if they can order that part right from Dometic/A&E.
2) See if you can find printing/stamping on the strut tube itself that shows a weight rating. It may be like 75#, or I suppose it could be more like 125#. Go to your local auto parts store (preferably with the old strut) and see if you can find one that both matches the weight capacity and the overall open/closed dimensions. The weight is definitely most important, but the open and closed lengths also matter so that it fits in there.
These gas struts are essentially the same as what's used on the rear hatch door of an SUV, and should have a similar weight capacity, so an auto parts store should have it unless Dometic used an odd size.