If you see 503 Service Unavailable Error in Apache Reverse Proxy Environment with the perfect settings, it might be something associated with server security - it can be solved by /usr/sbin/setsebool
If you see above message even though your apache settings is perfect, you can try below command on your terminal:
/usr/sbin/setsebool httpd_can_network_connect 1
By the way, the above settings may not take effect if you reboot your server. So what I do is turning SELinux off as
sudo setenforce 0