So, where does it go.

I bought an all singing dehumidifier just before the new year, and when I remember to turn it on, I set it for 8 hours. In fairness its been on for about 70 hours max since then, and I have it set up to drain into the sink. Tonight the sink was full to the top, I have been monitoring. So my question is, where does this water go, if its not caught in the machine. Incidentally, it blows out heated air keeping the van around 09° - 15°.

Unless you block up and seal the vents (fridge, cooker, water heater, and heater/airconditiong vents in the cab) a dehumidifier will just draw in moist air from the outside and extract the moisture from it. We have never put a dehumidifier in the motorhome.

For extended storage we do take the mattress into the house and we turn the seat cushions up on their ends and leave the cupboard doors open to allow air to circulate. We also leave the fridge turned off but with the door open. Everything is cleaned thoroughly before storage. Frequent inspections during storage take place. We have never had a problem

Understood, we used to do the same and it worked well. My point being, the fan in the dehumidifier wouldn't be strong enough to draw extra air in from outside, so that doesn't really answer my question.

ever seen a dehumidifier at a dealers?
no

best way is to open door and a few windows for hour or so a few times over the winter to let air flow in
once a dehumidifier has started working it will pull in more damp air through the various grilles and outlets in the van and in fact create more damp

if your going to use one then only put it on once or twice a week for a few hours only, not 24/7 for two months

to make really sure and maintain damp air free van just keep using it lol