If you have been RVing for any length of time you know how the tanks seem to never be the level the sensors say they are. Usually what needs to be done is a very good cleaning.
I’ve heard about dumping ICE down the toilet and then driving off, but I have a problem with that: It melts as soon as it hits the contents, so if you are just adding water (melted ice), use these tips I’ve gathered to help you a little.
Start with the Dump Valves:
Some RV’s have Black Tank Flush connections.
Here you would attach a garden hose (NOT the drinking water hose) and blast water into the tank while the valves are open. 
What helps with this is attaching a CLEAR elbow.
Here is one with a flush connection:
Another option is a Clean-Out Wand.
They are either STRAIGHT…..
Or FLEXIBLE:
Insert the wand into the toilet while attached to a water hose and RINSE from the top.
Here is the video to follow along:
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