route entries are getting updated on reboot
You'll probably have to dig around in /etc/sysconfig/network and /etc/sysconfig/network-scripts
The default gateway is determined by the network scripts which parse the /etc/sysconfig/network file first and then the network interface ifcfg files for interfaces that are “up”. The ifcfg files are parsed in numerically ascending order, and the last GATEWAY directive to be read is used to compose a default route in the routing table.
For CentOS 7 you may be looking for DEFROUTE directives in those files too see https://wiki.centos.org/FAQ/CentOS7