It is very simple to replace the black tank valve with a dual valve
and run a short pipe from the gray tank valve location to the new valve.
see my post about what I did here:
HW296 plumbing MOD
only used once this year but it worked great. pull the lever for the black,
then pull the lever for the grey to rinse the Rhino. One hook-up and one
size hose. And no cross contamination. Mod was less than $50 and took
about 45 minutes start to finish.
here is a link to the valve I bought from amazon: