As netjam said--there are several valves available that will attach to the drain line just like the hose, and you can use that to stop the 'baths' you get until the upstream valve can be replaced. I have one on my Suites permanently--cut into the 4' line, glued a valve in, and now have a failsafe closer in case the others fail.
Having a problem such as yours can sure take the fun out of RVing...
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired