newbie questions on black water tank and campgrounds

To avoid issues of the dreaded pyrimid in your tank, yes keep the black tank valve closed until 3/4 full. What this does is allow the paper to partially desolve and keeps the tp from forming a pyrimid at the entrance of the drop from the toilet. You do not want that to happen. Dont ask me how I know.

Keeping the valve closed allows all that is flushed to float and not harden. Then you can dump it at 3/4 full and it will drain without issues.