PMcColley wrote:
Do you all use bar soap or shower gels when using shower? Bar soaps will leave residues in the tanks that settle out of the water and build up into a large sheet overtime like your describing due to the paraffin ingredients. You can see in pic below the soap buildup from a shower that wasn't cleaned over a long period of time. Combine the soap with the dirt, body oils and minerals from the water and its easy to get a buildup over time.

One trick I have seen used on black tanks to break up clogs and help clean tanks is dumping 40# of ice cubes in the tank and going for a drive, as the ice bounces and slides around it knocks some of the crude free. Doesn't work every time but its a cheap tool that only leaves you with water to drain. Of course this wouldn't work in a grey tank.
That shower is disgusting! I certainly hope that's not from an current, actively used motorhome? DW would pass out cold if she saw a shower like that. Someone needs to hone their cleaning skills. :E