I am guessing you have the 30/35L factory fit tank filled from the outside via a petrol cap type filler.
I am on my 3rd van like this and use the following method.
- close all internal drain valves.
- add a couple of watering cans full of water.
- turn taps on to bleed system and get water running freely, through both hot and cold taps settings.
- continue filling up tank until you hear it glug and overflow and spurt out over your foot.
- put bucket under the waste outlet.
- when bucket is full of wastewater add another can full of water to the tank.
- two days before leaving site do not bother filling up the tank.

Alternatively you could fit a water level sensor.

Colin

These people are very helpful and should be able to provide the individual bits you need. I have used them for all my extra 12v bits. https://clearcutconversions.co.uk/produc...-indicator-kit/

Colin