Hi ahlsner, a couple of small notes:
1) There's probably no need to set auto_ifconfig=YES in
/etc/rc.conf (re: your first post), because the default value is set to YES already in
/etc/defaults/rc.conf. See also the man page for
rc.conf(5).
2) If you put the ifconfig command line into a file, the file's name is
/etc/ifconfig.rtk0, with a dot "." not an underscore "_" . You can also put it into an environment variable ifconfig_rtk0 which has an underscore (as in your second post, though it wasn't clear if you meant a file or a varaible). (I've only ever done it with the file myself.)
Once you've got part 2 set up right, then on the next reboot the interface should be configured how you like it, including the correct IP address.
Let us know if it works or not.