@Thad You need to add it to your nginx.conf to actually log the error and access portion of the proxy.
For example:
server {
listen 443 ssl;
server_name cloud.your-domain.com;
error_log /var/log/nginx/owncloud.access.log;
Then you can just do a service rockstor restart
to re-initialize nginx and it will (should) start logging. That is, if you have everything else in your nginx config set up right.
I did a how to using letsencrypt, subdomains, and nginx to pass all my owncloud and other Rock-On containers through HTTPS/SSL.