In brief
Fresh water is drinkable and goes to the tap, the shower and the toilet flush; grey water is from the basin and the shower; black water is from the toilet. Almost every winterisation problem is caused by water left in the system rather than by antifreeze, so drain the system, then blow it out with air, then, unless you have a sealed antifreeze fill, add antifreeze to the remaining traps and pipes.
How much water do you actually get through?
A short shower uses around ten to twenty litres, a hand wash one to two, and a toilet flush between three and six depending on the model. Two people on an ordinary day typically consume eighty to a hundred and fifty litres if they shower every day, and far less if they use the facilities ashore. Before a wild-camping trip, work out your consumption per day, multiply by the days between fill-ups, and compare it with your tank size. That number, not the tank size, is your real range.
The plumbing you can and cannot drain down
A well-planned RV lets you drain the fresh tank, the grey tank and the toilet holding section in one direction, and keep water in the boiler or the fresh tank if you are using antifreeze. Before you start, learn which valve does what, because the layout is not the same on every model.
Winterising an RV, step by step
Drain the fresh and grey tanks and flush them. Open the low-point drains and the tank outlets and let everything out. Blow the remaining lines out with low-pressure air, starting with the fresh side. Add antifreeze to anything you cannot drain, or to the whole system if the manufacturer prescribes it. Remove the water from the pump and the filter housing, and take out the shower head and any inline filters. Finally, do not forget the external hose, the hose reel and the container you filled from. People forget the external hose every year, and every year it splits in April.
Smells and sensors
Persistent black-water smell is usually a dry trap, a vent that is blocked or perished, or a tank that is not fully draining. Grey tanks are not designed to hold water for long, so never leave a grey tank standing. If the sensor readings stay stuck, the sensors are typically coated and need cleaning with a tank cleaner rather than with a jet of water.