Robert,
The overfill valve/lint-filter is designed to pull off for cleaning, which I do at least onece a day. Capping the vac pipe makes sense when pressure-washing the waste tank - I find the plastic cap of a aerosol can fits well.
For the pressure wash I have a male quick-connect with a male-threaded tee jet screwed into the hosetail thread. This simply attatches to the solution hose in the normal manner and I find I can reach even the most awkward corners with ease.
If the tank coating is being obstinate, I have a loo brush, with cut-down handle for this.
Whilst tank cleaning, use a cylindrical bottle brush, from inside the tank, to remove the fluff and grit from the bottom of the gate valve slot - this will allow the gate valve to fully close, preventing dripping, and air ingress whilst running.
John.