I've been through the same problem some time ago, even with the same misleading syslog message. Turned out the core file was not where the logs said they were put, but in the exact working directory where I manually started the service.
Try running rtpengine without the init.d scripts. Rtpengine has its way of daemonizing itself anyway.
If you are unlucky even after doing this, maybe a standard shell find could help you.
# find / -name core.<pid-reported-in-the-logs> -type f
# find / -name core.1123 -type f