Most of today's RV sidewalls are of sandwich construction. If they will bore out the holes, install metal dress plugs with two part epoxy, I see no reason not to take off the 13 footer and install the correct awning. I know that our 16 foot awning sometimes doesn't feel big enough.
If you feel they can't properly fill the holes and you don't want to kill the build, this may be an option you can live with. Have them install an awning two part zippered sun shade like this:
ShadePro Inc.: Vista Shade for Electric RV Awnings
I added this purely as an example...no affiliation.
Dang, we pay too much for these toys for such mistakes...frustrating!