Flush valves for the black water lines.

When upgrading to PEX I found the plumbing on my 1969 Airstream to be quite interesting and even a bit perplexing a couple of times. One item that I haven't quite wrapped up yet is what appears to be the drain point for winterizing. I'd like to use this for dual purposes I want to use it as a sewage hose fresh water flush to run fresh water through the hose to clean out the unpleasantries. I'm not keen on allowing a contamination point into my potable water system but don't want an extra hose to make it work. Could I finish this off with a check valve in addition to my manual flush valve and still stay sanitary with it permanently hooked up?