Timely Holding Tank Evacuation

I recently purchased a 2001 Jayco motorhome and the problem I have concerns the amount of time it takes to drain the gray water tank. The black water drains very quickly but when I drain the gray water the bulk of the water drains quickly but it take several minutes (up to 10+) of a steady trickling to drain completely. I've had the tanks flushed and scoped and I've personally inspected the drain valves and they are fine. I've also driven the right front tire up on blocks to help but nothing seems to work. Could you please give me an idea as to what the problem is? This is rather bothersome when others are waiting in line behind me. Carl, (Renton, WA)

Barring any severe blockages inside the holding tank Carl, it is apparent a venting problem exists. As a holding tank drains, air must enter the tank from above. All holding tanks must be vented through the roof of the RV. In some cases, the vent pipe can fall down inside the tank, immersing itself in the contents of the holding tank. This effectively blocks off the vent, thereby eliminating the venting action of allowing air to enter as you try to empty that tank. This is why it seemingly drains normally at first; the weight of water forces the initial gushing. Venting allows air to enter the tank.

Here’s an analogy….place a drinking straw into a glass of water. If you simply lift the straw out of the glass, the water inside the straw drains out as you lift. Now place your finger over the open end of the straw and lift it out. The water remains trapped inside the straw until you remove your finger from the end. Liquid entering the holding tank must displace the air when draining into the tank and air must enter behind the contents during draining of the tank itself. That vent through the roof must work in both directions.

From up on the roof, remove the cover for that holding tank vent. If the ABS piping is not protruding above the roof a couple inches or so, chances are the vent has slipped down into the tank and made contact with the contents. In some cases, you can re-attach it correctly from inside the RV; in other cases, the holding tank must be partially dropped in order to repair that vent connection. A thorough inspection will reveal the best method. The other possibility, albeit slight, is that the manufacturer plumbed the drainage system without the requisite slope; water cannot flow uphill. This may not be your problem since you’ve raised the coach to try it, but it may warrant a closer look.