Had a similar thing happen to me on my firts ever trip.
WE resolved it by the use of silicone spray.
If you've got an awning then maybe you have already invested in a tin of silicone spray to lubricate the awning channel before threading through the awning bead?
If you don't have silicone spray then spray furniture polish e.g. mr sheen etc, will be adequare - its the silicone containing stuff you need.
Spray the affending area of the zip teeth with the silicone (polish)a nd maybe rub it evenly over the a cloth. Now attempt to pull the zip back over the area that has come apart - this may need some effort. The silicone makes the teeth 'slippy' enabling them to be forced back together.
If it don't work you haven't lost out on anything, if it does you saved yourself having the zip replaced.
We did this for the entire zip length of our 980cm (9.8 metre - sorry but you never now) long awning which the other half pulled apart thinking it would solve the samll problem only then to create one big one.