It sounds like you are fairly new to the RV / Camping experience. Let me give you something to think about, that if you start practicing now and do it all the time, will always benefit you.
I am assuming your black and grey water tanks flow to a single outlet. If they do, then this is very simple, and the benefits are tremendous.
First, you need to purchase a Flush King type of valve that attaches to the end of the sewer outlet. A Flush King comes with a clear section of pipe so you can see what is being discharged from the tank. I always dump with a clear section somewhere in the hose line so I can see what's actually dumping, and how clean or dirty the tank still is.
So, with the 3rd valve attached, you can now back-flush the grey water tank into the black water tank and fush the black water tank out. The benefits are, the back flush works. It will pull a lot of "stuff" that sticks behind. Also, you are re-purposing the gray water a second time (which is going to be dumped anyway). And third, you do not need to add extra water to your black tank (via the toilet) to add to flush it out.
Here's how you do it.
3 valves.
Valve 1 is the Flush King.
Valve 2 is the Black Tank
Valve 3 is the Grey Tank
Open 1 and open 2 (Flush King and Black). Leave the gray closed. Dump everything out of the black tank until it quits running (you can see this with the clear section of pipe).
Now, close the Flush King (1) and open the grey tank (3). The black and grey are now both open. The Flush King will not let anything drain down out. When you open the grey valve, the water will back-flush into the black tank. Because the black tank is empty, nothing will force itself back into the grey tank.
Leave the grey tank valve open, count to 10 or 15 and then close it. This now closes off the grey tank so nothing will go into it.
Now, open the Flush King (1) again and let the black tank drain. Again, watching that clear section of pipe, you can see all the extra 'stuff' coming out that did not exit on the first dump.
When it quits flowing, repeat these steps again. Depending upon how much water you have in the grey tank, you can back-flush a couple times real easy, Sometimes, I can back-flush 4 times. I usually strive for 3.
Each time you do this, you are lowering the grey tank 1/2 the starting capacity.
So if there is 20 gallons in the gray tank to start with, 10 will transfer (back flush) to the black tank. Next half of the 10 will back flush Leaving 5 gallons). That will be enough to do a final rinse of the sewer hose. So the secret to flushing 3 or 4 times is to have a full tank of grey water.
If you do this EVERY time you dump your tanks, your tanks will not have odors, will always be flushed out good, and you'll never have any poo-build-up inside your tanks.
Do this when you break camp also. Then add a final 5 gallons of clean water back into your black tank and hit the road. The sloshing while driving will also help clean the tank out. Add a 1/4 cup of Dawn Dishwashing liquid, and your tanks will never, never smell.