Dump valves?

[shields71646]

I bought a Sandpiper 36.5 earlier this year. It being used I am short on some of the manuals that most likely came with it when it was first purchased.
I am having a little trouble figuring out the dump valves. I believe I have a galley holding tank, a grey water holding tank, and a black water holding tank. Someone please correct me if this is wrong. Looking at the dump valves under the camper I see 2 dump valves on small lines that I assume to be the galley and grey water valves. Where I get lost is where the large line, i'm assuming the black water line, comes out of the tank I have a dump valve right under the tank. I also have another dump valve on the same line before it gets to where the grey water lines y's in. I don't understand the purpose of the 2 dump valves on the same line.
Please tell me if I have it all wrong. Any advice or pics would be appreciated.

[commander ryker]

when you have dual grey tanks its common for the manufacturer to put 1- 1 1/2 " gate valve on each tank plus sometimes a 3rd one on the longer line so that it does not let water get into the line while you travel. if you look at it like this, if they did not put a valve close to the tank (one farthest away from the dump point) and then another at the dump point, otherwise the line would always be full of gray water and could break off due to weight while in transit.

[turbopuppy]

It is set up like that to isolate each tank and its drain line while you dump one at a time. You would probably want to start the dump with the black tank first. First close all valves then open the valve at the end of the drain lines where it goes into the sewer hose. THEN open the black tank isolation valve. when that tank empties, close the blank tank isolation valve . Open the gray tanks one at a time. This keeps the black water from flooding up into the grey tanks whould the grey levels be alot lower than a full black tank.

[shields71646]

Thanks for the tips. We are brand new to RV's.