Originally Posted by hockeytownmom
.. Remind me on the DIY fix?
On an empty black tank, 1 cup Calgon disolved in some warm water, another gallon or so of water before your travel - I do this religiously.
Reading the black and grey tank is a crapshoot - pun intended - you have to accept that - it will never be that precise, you will get a feel when they are really full after a while. Using Calgon it will read right most of the time, but not all the time.
Horst probes seem to work well, as Lou can attest.