Could still be a tank flush... Most factory installed tank rinsers go first to a "high point" (most often the bathroom sink) where there is a plastic "backflow preventer" installed. That keeps the possibility of a siphon effect pulling effluent from the tank up that line and out the side of your trailer... Sometimes those backflow devices fail and won't let the water flow through to the tank rinser. Look, again, under the sink and see if you see such a device in the line. If you do, and hooking water up to the line outside does squat, it has probably failed. You can get a replacement unit almost anyplace.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)