If it was an inch or two, it might be a cost savings.
We used eternabond on a slide topper and got 4yrs out of it but it was only a 2" tear. At 12" I wouldn't expect the same life.
Also, what is the condition of the canvas? Our repair has finally given out but that is because the material is failing not the tape. I'm not sure what a full awning would cost but we found replacement fabric for a large slideout for around $160. Replacement looks pretty simple (we'll find out when the weather warms up a bit more)