I think it is better to start up RTPProxy first and then Kamailio (because after startup Kamailio will try to connect to RTPProxy). A good example for how things should be, can be found at the following github repository which contains Ansible scripts for automatically installing and configuring an Active-Passive Kamailio cluster; these scripts allow you to choose between installing a local instance of RTPProxy or a cluster of RTPProxy servers :

https://github.com/ghrst/Kamailio-HA