I have an octane 265 but yours shoukd be similar. I have a seperate valve for the black and gray. One of them is tucked up by the frame of the trailer and is hard to get to. Mine are labled on the outside of the trailer. Hope this helps.
I have a 272. My gray one is very easy to see, the black one though is tucked up into the piping really well. You have to get down on a knee or two and really search around. Good luck!
I got tired of having to get on my hands and knees on the wet ground to open the grey water valve I put electric valves on both. Hard part the 161 has no outside storage to put the switches in.