A camper carries three tanks and all three need attention: fresh water in, grey water out, and the toilet cassette out. Getting into a rhythm with them is most of what separates a relaxed trip from a stressful one.
How long a tank lasts. With 100 litres of fresh water, two people washing up and having short showers get roughly three days. Grey water fills at about the same rate, so the two usually need doing together. A cassette is typically good for three to four days for two people.
Where to do it. Campsites all have a dump station (tyhjennyspiste) and will usually let you use it for a few euros even if you are not staying. Many larger service stations — ABC and Neste in particular — have one, and they are marked in the usual camper apps. Some municipal harbours and marinas do too.
What not to do. Grey water goes in a dump station, not on the ground: it carries washing-up liquid and food. The cassette goes in a dump station or a proper WC connection, never in nature. Both are the fastest way to get overnight parking banned somewhere it was previously tolerated.
Winter. Below freezing, drain the fresh water tank and lines whenever the camper will stand unheated, and refill when you arrive somewhere. Our campers are winterised, but a frozen line is still a frozen line if the heating goes off for a day.
Fill up before you leave a service point rather than when the tank runs low. In sparser parts of the country the gap between stations can be a hundred kilometres.