This may be a repeat of some older threads, but it's always good to review these things from time to time. Anyway, I found a video on YouTube that, in my opinion, is probably the best information and advice about dumping and cleaning your black that I've ever seen or heard. I fully agree with just about all of his comments and recommendations.
Just a couple of comments before you watch the video.
First, most Sunlines don't have the black tank flush kits installed. Basically, they're a rotating or oscillating sprayer inside the tank that effectively cleans the walls and ceiling of the tank with pressurized spray. These kits can be purchased at any RV dealership and are fairly easy to install. My Cougar came with one, and I highly recommend its use.
Alternatively, since Sunlines all have a straight, short drop from the toilet into the black tank, one of the wands will do a very good job of cleaning the walls of the tank. When we had the Sunline, I used that every time I dumped with great results. It is a good alternative to the flush kit, and less expensive. My Cougar has a 3' drop from the toilet into the black tank, so the wand is not very useful unless I extend it with some plumbing parts from Home Depot.
Secondly, lets talk about the ice cube trick. It's long been conventional wisdom and advice to toss a bunch of ice in the black tank and take a road trip. The theory is that the solid ice will act like a gentle abrasive and scrub off the debris on the walls and floor of the tank. There's a guy on YouTube called The Fit RV
who has posted a number of videos examining a lot of these RV maintenance ideas and systems. He puts them to the test and shares his findings. In the case of the ice cube trick, he found it to be fairly good at cleaning small debris from the floor of the tank, but only marginally effective at eliminating the dreaded black pyramid. He also has a video testing the different tank flushing options. If you plan to add one to your rig, it's worth a watch before you spend your money. Luckily, my Cougar has the one that he found most effective.
Here's the video: