You would be much better off figuring out ways to replace /etc/rc.local than to perpetuate it. In most cases I've seen, there are already existing constructs to take the place of what people are doing with this file (udev rules, tmpfiles.d, modules-load.d).
Then look at the journal for errors... You'll need to look at messages for the PID that systemd-load-modules ran as. Messages will not show up for the service. 'systemctl status' shows you the pid, and then you can run 'journalctl -b _PID=$pid'