Thanks all for the replies. I think I can conclude that I don't have a problem with the drainage, and I hope the video helps others in the future.
With regards to those tanks with dirt, in case it helps, in the two T4 Californias I have had, for one reason or another, I had to dismount the entire kitchen/sink/fridge cupboard completely in order to get to the waste water tank and repair it. On both occasions, what I did to clean the tanks was to pour 4 or 5 litres of bleach (the one that you can use to potabilise water), and filled the rest of the tank with water, left it for 24 hours, and emptied it. This removed all the dirt and left them completely clean. Before taking the picture above, I did the same on the Ocean. The only difference is that this time I was not able to see how dirty it was before doing it, but with the T4s, I remember the tanks were pretty filthy, and the bleach left them perfectly clean. In the fresh water tank I always did the same, more or less once a year, to keep it clean, as I drink water from it.