So back to the subject
I see what you are wanting to do, but I'm not sure you fully understand how holding tanks work. Just a off-the-cuff guess, but I would imagine if you did leave your black tank valve open (usually a bad idea) you would need at least 10-12 gallons of water to flush a bowel movement completely from the tank. even then, I'm not sure that would "doo" it
Holding tanks work by suspending the solids (poo and TP). The chemicals you add assist the water in flushing all the solid waste from the tank when you dump it.
I have attached a PDF that outlines holding tank do's and don'ts It also has the holding tank chemical "Home Brew" a lot of us use.
I DO understand what you want to accomplish, but I think you will be a much happier camper if you use the toilet that came with your camper (I DO recommend ditching the cheap plastic seat and installing a residential toilet seat).
If you have concerns about solid waste adhering to the toilet bowl, just install a hand sprayer:
Finally, a lot of us on the forum swear by the Blue Streak:
Chemical Dispensing Systems for RV & Marine | Blue Streak LLC
(Even if you use the Blue streak, still treat your tanks with the home brew mentioned in the attached PDF)
DO NOT attempt to put the home brew in the Blue Streak! Don't ask me how I know this