If the strap is the worst of it, you definitely don't need to replace the whole awning. There's also a good chance the strap is tucked up inside the awning and just not visible right now.
You can buy new straps at an RV dealer or Camping World. They are pretty standard, just get the one by A&E to match the awning. A universal one or another brand may work too if the A&E isn't available. Just make sure to not buy a "window awning" strap- those are much shorter.
Once back to the trailer, pull down on the release lock on the front of the tube, toward the front of the trailer. Then loosen the inner awning arm nut- a big black nut near the bottom of the awning arms, on the back side. Just give it a few turns loose, don't go too far. Then unsnap the inner awning arms with the black clips- make a "C" with your thumb and finger, put it around the awning arm, squeeze together on the black snap part, and push back.
At this point, you are ready to pull the awning down with the strap that you don't have. So- ideally with two people, grab the outer awning arms and pull out. The inner arms will stay there, so you may need to work your fingers around to just grab the outer part. It's harder to describe that it is to do. The awning may be stuck a little bit, but keep pulling on those arms until the awning is completely unrolled.
Once unrolled, there should be a slot in one end cap of the awning tube, and channel in the tube that lines up with it. You can slide your new strap into that and roughly center it in the tube. If you found an old strap or part of a strap, you can similarly remove that one through the same slot.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR