These two tubes are coming from under the bathroom sink cabinet. One side of these tubes go to the faucet and rest of the trailer. The other side (pictured here) go through the floor to the undersid...
OPs link Red is hot and blue is cold looks to be low point drain that should be capped or a valve installed. It should have water flow while using holding tank and pump. Put some water in freshwater tank and turn on pump to verify!
