The short answer
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 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 use?
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 a normal day typically consume eighty to one hundred and fifty litres if they shower every day, much 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 with your tank size. That number, not the tank size, is your real range.
The plumbing you can and cannot isolate
A well-planned RV lets you drain the fresh tank, the grey tank and the toilet holding section one way, and keep water in the boiler or the fresh tank if you are using antifreeze. The components that fail first when water is left in are the fresh tank outlet, the pump, the water heater, the toilet cartridge and the grey tank sensor. If you cannot fully drain something, circulating antifreeze through it is the answer; plain antifreeze in a tank you still drink from is not acceptable for drinking water.
Winterisation, 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 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.
Smell 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 sensor readings stay stuck, the sensors are typically coated and need cleaning with a tank cleaner rather than a jet of water.