I find Thetford manuals to be almost impenetrable!
Especially the part covering the filling of the flush tank.
At last, a clear and simple guide!

https://www.youtube.com/watch?v=dF4nCM6Zkbc

Thanks to Costock caravans!

That video is very good, except for their using the Thetford pink flush additive - basically, don't! It gunges up over time and becomes black bits in the water.

Use Elsan Organic instead for flush and holding tank.