Heating water with a device powered through a cigarette lighter socket is very slow. I had a very small capacity 12v jug (made 2 tiny cups). It was virtually useless and I binned it.

Typically, a cigarette lighter socket has wiring and fuse for a maximum of 10 amps, maybe 15 amps. So that means a maximum of around 150 watts. A 150 watt element would take about 5 minutes to heat 1 litre of water by 10 degrees or 250ml by 40 degrees. You probably want the water to reach at least 80 degrees so it does not cool too much when you add milk.

The problem is as the water temperature rises, it gets cooled by the air and surroundings. So at 80 degrees the heating element is making very little headway. My guess would be about 20 minutes to heat a single cupful. Add instant coffee and milk.

If you want something that is more sophisticated a 12v cigarette lighter socket is not the way. A portable butane stove with a plunger to make real coffee would be my choice.

Or a 240V inverter.
It could be connected to the crank battery and only used when the engine is running if you don't have a big enough system in the van.
Cheers,
Peter

A stainless steel Bialetti Musa Moka pot on the cooktop is our choice, & we've tried most methods.

Or, get a generator. Pull the cord and "hey presto" coffee machine in action. Plus it is available for any other 240 appliance you need. Zimple. But move pretty quickly, because I understand they are going to be banned, one day :)