I can't speak to the water tank replacement but on my 2003 the seat is easy to remove. Just some #2 square drive (i.e. Robertson) screws top and bottom and lift the seat out.
I suppose there might be differences in seat mounting over the years but few things are complicated on the Casita
. If are differences you might report back after the job's done for our education.
There's a cleat screwed into the floor that the bottom of the seat screws into. There's s simple flange at the top.
I took mine out to put a water pressure accumulator in the other back corner and was pleasantly surprised how quick and easy the seat removal was. I'm including a picture showing the street side back corner just to give you an idea of what the seat is attached to.