Like many above have posted, factory tank sensors almost never give you an accurate level of your tanks, except maybe the fresh water tank. On my current RV (2007 Montana 3400RL) I installed the Tech-Edge Mfg “iSeries Kit” and love it. Yes it was an effort to install but it gives me an accurate percentage of how full my tanks are and warns me when they get to a preset “full” or “empty” (fresh water) level.
The system is not expensive ($229) and works very well. You mount a special tape on the
outside of your
non-metallic tanks and run 2 wires from the tape to a control panel mounted inside your RV. Every few minutes it automatically displays the percentage full on a screen or you can view the percentages manually whenever you want. I love it.
Get more details from their website,
www.tankedge.com.
With regard to dumping my tanks, I use a 45-degree clear hose fitting at my RV sewer connection so I can see what’s draining out of my tanks, especially the black tank. When I drain my tanks, I try to have them on the fuller side if possible. I start with a little of my galley tank to wet the drain hose. Then I close it. Then I open my blank tank valve and turn on the power flush. You can often hear it spraying the inside of the tank. When it seems like most of the ‘black’ is out (water starts to become more clear), I turn off the power flush and let the black tank drain to a dribble. I close the valve and open the power flush again for a few minutes, then open the valve. I do this a couple times to get most of stuff out of the tank. When the water is quite clear, I close the black and then drain the galley. I drain the grey tank last to ‘wash out’ the hose. I haven’t used tank cleaners in years and rarely use deodorants/tank solutions in my tanks. I feel they are unnecessary. I have not had any problems with this routine. I don’t try to get every last drop out of my tanks until I winterize my RV.
By the way, after I dump, I disconnect the drain hose at the RV first and lifting it up, compress the tube to help drain any liquid on ribs on the inside of the hose. It’s amazing how much liquid comes out of the drain hose. Then I store the hose.