The SIP should be easy to set up. Give it the device pool and SIP settings for a normal gateway.
The only real thing you need to worry about are these things:
-Make sure Rightfax is sending the SIP to an IP address and not a domain name
-The remote IP address at the bottom of the trunk is the source IP of Rightfax
-The Calling Search Space assigned to the trunk has access to the right partitions to call what you want.
The routing of your DIDs is going to be done through a route pattern that points towards your SIP trunk.
-nick