To be somewhat delicate about it (and that is not my strong suit) - you are looking down into the tank when you flush... you will know when it is getting full.
We have been RVing for over 30 years, and the SeaLand Marine/RV toilet in the later C-Dorys is easy to use, especially compared to a more typical marine toilet. A few things that have worked well for us:
* Use rapidly dissolving toilet paper (Angel Soft works well and is easily availablel).
* Consider putting a ball valve into the water line feeding the toilet - using the foot pedal will fill your tank faster when you can't control the flow of fresh water into the toilet.
* Use liquid RV black tank products without formaldehyde - it will keep the odor down and break down the solids faster.
* We used a squeeze bottle with fresh water in it as a rinse instead of the foot pedal... it really saves on the tank "storage."
* After a pump out, run more fresh water into the holding tank and pump it out again. Put the RV holding tank stuff in with a bit of fresh water when done with the pump out.
* We used the shower in the head on our CD-25. Water will get into your toilet bowl from the shower. My wife made a waterproof cover to put over the toilet when we showered - again, saving that holding tank from filling faster than necessary.
* All this sounds like a bunch of work - it isn't. Most of the time, with two of us on the boat full-time, we needed to pump out about once a week +/-.
Congrats on the upgrade to the TC255 - great boat!
Jim