False readings. Fresh/waste water levels

I received an alarm from Sargent Autosleeper monitor.. It said temp below zero and you have water in your tanks. I had  drained all of the tanks in November. The readout on the electronic controller shows 25% fresh, 75% waste. The tanks are def empty and flushed through.

2020 Warwick Duo. Not sure about the control panel. EC700?

Any thoughts? Thanks as always.

Great van but there are niggles! There's aye sumthin!!!

Our van invariably shows the waste as full or partially full these days.  It all worked fine when new, but I suspect remains of dirty/greasy water just hangs around the sensors and gives a false "full" reading.  It also worked fine for a while after I gave the tank a proper clean, so I don't think it is a failed sensor (on our van).
I don't have such a good explantion for your fresh water tank showing 25%.
On our van the lowest part of the tank is front right, so I would just find some suitably sloped ground depending on your tank layout and empty the tranks.  You should be able to feel convinced that they are empty after that.

Possibly the seal on the outside of the sensor isn't quite 100% and damp has got into it and is giving a false reading.  My waste water always gives a contents reading of around 25% when it rains, just haven't got round to checking the sensors yet.

The sensors on the outside of the tank are possibly damp. Happens on my Symbol when the roads are wet, fine when dries out. I need to clean and re-seal the sensors when weather permits. 

It obviously happens a lot as it's on the A/S website....

Was the temp below 2ºC when the alarm was sent?  If it happened when the tanks were empty it suggests you don't shutdown the EC700 PSU when the van is out of use so the sensors are permanently on.  It has been suggested on the Forum that this contributes to their unreliability by wearing them out but I don't know if this is true.  Shutting down the EC700 would stop the frost alerts being sent.  The problem with your sensors is most likely caused by damp between them internally and/or externally.  A good clean and descale of both tanks, applying sealant to the outside of the sensors and checking their spade connectors with the harness are clean and dry should all help.  A similar problem was reported here:

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It might be the damp weather.

Thanks caraman and everyone. All interesting and helpful.