Water questions

How does that sit with MPG, or is that of no consequence because of the shortage of water on your continental travels CY.

A full water tank adds a very small percentage of extra weight. Of course, it depends on the van, but smaller vans typically have smaller tanks so I imagine the percentage would not vary a lot. 

I am no expert, my mechanical knowledge is zilch, but when I have seen this discussed elsewhere, over many years, the conclusion seems always to be that weight saving at this level (probably less than 3% on average) will make a negligible difference to fuel consumption.

I rarely travel on a full tank of water, but I do always carry some water - usually around 50% full. I very rarely travel with anything in the waste. Any steps beyond that are just not worth the hassle. 

The difference between an empty and full tank is less than 3% of the van weight. A small extra load when accelerating but since the biggest bit of fuel consumption is air resistance, not worth bothering with. I'm getting an average of 32.4mpg over the past two years with my A Class which, judging by the "bug splatter" is not that aerodynamic.