You dont . Hook up the gray to the sewer while you are camping. Then once the black tank is full then hook that one up and dump it. You do not want to have the black tank draining to sewer as you camp as solids will build up in the tank, so you only want to dump the black when it is at least a three quarters full. If it is not at least that full, add water to get there. If you are interested, some people have done a mod to connect the two outlets into a single, that way they use stored gray to rinse the hose after a black dump. Search for that here on the forum. Have a great time camping....
HTT: "EscapeII" 2016 Shamrock 23WS (current)
PUP: "Escape" 2010 Rockwood HW 277 (gone)
TV: "Bruce" 2011 Dodge Durango V8 Hemi Crew
Just us gals (me, Sis and our daughters)
Traveled 19520 miles and 209 nights in 7 years and counting