Washer water goes in to black tank.

There is NO RULE or Code that states that all Gray water goes to the Gray tank. Odds are the design and floorplan of your specific RV required that the Washer drain be plumbed to the Black tank, because there was no way to run the plumbing to the gray tank. As to your question about the replacement of the Valves, there is NO WAY by replacing the valves, that, that would reroute the existing drain pipes to another tank. BTW, there are SOME RV's that the washer drain does NOT go to any tank. The drain is run to a 3 way fitting at the dump cap area and you MUST be connected to the CG drain system when running the washer as the drain water flows direct to the dump hose. Doug