Please let us know the year, make, and model of your RV and I'm sure someone will be able to give you some great/specific info.
I have a 2015 Thor Majestic 28a Class C RV. I had a leak recently on my black waste tank line (from the toilet to the waste tank where it connects). I also had to deal with the waste line from the bathroom sink to the black waste tank in the process. I posted a detailed description of the project with photos of every step. It shows the steps involving the sink line going into the black tank so maybe it will give you some ideas.
https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/30326133/gotomsg/30326133/p/1.cfm#30326133
In the above link, scroll down to post ~#10 for the detailed description with photos.
The key piece of info that would have saved me hours (and might save YOU hours) is that the job required removing the rear wheel well insert. That gave me complete access to all the pipes going to the black waste tank (....maybe YOUR RV will be the same).
Step #1: Clean out your black tank as best as you can before you start!
Let us know. Good luck!
Chris
