Update Trouble of the Worst Kind.Black Tank Won't Empty Help

I think the dealer who talked you out of the Flush King was thinking of using it with the valve open to just squirt water up the pipe. That would only rinse the pipe as far as the water could reach.

I closed the flush king outside valve and opened the tank valve which allowed the pipe to fill with water and become pressurized.

That will put the full water pressure on the clog. Total pressure applied is the water pressure x the surface area, so on a 3" pipe you would have quite a bit of pressure.