Depends what the awning is made of. I've heard of people using eternabond, white duct tape, vinyl repair kits among other things. Most repair both sides if using tape.
Camco makes a tape specific to repairing awnings.
I've not had the need to repair an awning so I can't really tell you which is best, only that I've seen posts from people using all the above. Might be a good topic to search the forums for and see if there are any threads discussing the repairs to get an feel for their success.