I have the black tank flush and the "Flush King". I use the black tank flush for a quick cleanout and usually use the Flush King at home to backfill and get the toilet paper out and get the guages working. A word of caution. I was backfilling the tank and got distracted overfilling the tank and popping the top open. This now allows me to be an expert on taking the underbelly pan off. There was about 40 self tapping screws holding the bellypan in place. After removing these and doing some wiggling the bellypan and insulation dropped down. Looked like R12 fiberglass insulation in the pan. My black tank flush was on the black tank on the side between the tanks and on the drain side. I had to cut the plumbing and drop the tank, epoxy it and put it back up. Took 2-3 times as long to put the bellypan back on as I had to line up all the screws. I would guess a day would be lots of time to install the tank flush with a little help to drop and replace the bellypan.
Brian