If you are on full hook-ups, then you should have no problems with your tanks because you can just dump them on a regular basis. Figure 2 or 3 days between dumps and just do it. You sound like you use your black tank flusher, do so. When you start dumping the black tank, turn on the black tank flusher at the same time. It doesn't matter how full the black tank is. Contents will drain and the black tank flusher will get most of the remaining at settles on the bottom and push it on down. I've never closed my black tank valve and let the black tank flusher run, unless it's just for a few seconds to get a bit of extra to help push out the sewer hose.
Here's another tip and one that works very, very well. I don't know why folks don't do this, it's easy, saves water, doesn't take any real extra time, and did I say, works well.
That would be "backflushing". How do you do it.
First, you need a 3rd valve on your sewer outlet. This is assuming your grey and black water flow to the same discharge outlet. I refer to the 3rd valve as a "Flush King" but there are other generic versions that do the same thing: Click here.
You can also find these at Wall Mart, and all over Amazon.com, or any RV dealership parts store.
Attach the "Flush King" and now drain the black water tank completely.
Close off the Flush King (leave the black tank open) and now open the grey tank. This causes the grey water to back flush into the black tank.
There is no fear of black water getting into the grey tank. First, the black tank has already been emptied. Second the grey tank is fuller and will force grey water only 1 direction.
Wait about 10 - 15 seconds and now close the grey tank valve. This prevents anything from the black tank to enter the grey tank.
Now, simply open the Flush King valve again and watch the magic through that clear section of drain pipe!
Depending on how much water you have in the grey tank, you can repeat this 3 or 4 times. Each time, more and more stuff will pull out of the black tank.
Now that you completed this exercise, turn on your black tank flusher for few seconds and let it drain as it runs. Often times, it takes only about 30 seconds with the black tank flusher and water discharging from the black tank is now almost clear.
After you are finished with the black tank, the release the remaining water in the grey tank to flush down hose.
If you have a laundry washing machine, this process also forces laundry water into the black tank, which really helps keep it cleaner.
This process works well. It forces water into the black tank from the opposite direction. The big thing is, is repurposes your grey water and you use less fresh water.
If you do this EVERY time you dump the black tank, it doesn't matter if you dump the black tank every hour or once a week. That back flushing really renses the tank tank out. You do not have to wait till it's full. That's why I said, do it on a regular basis, every 2 or 3 days, and you'll never have to figure out how full the black tank really is.
However, you do need adequate water in the grey tank to do this. But even a grey tank that is half full, still does a great job.
When convenient (mostly when I winterize), I'll use the wand down the toilet bowl.
Again, I don't know why more folks don't do back flushing from the grey tank. It takes only a couple minutes longer to discharge the grey into the black, but both need to be emptied anyway. And the benefits far, far, far exceed the couple minutes it takes to wait for the grey to back flush into the grey (maybe -- 15 seconds for each backflush?)