Erwin: I have read the manual for the European Airstream series and they are quite different from the ones here in the US. The frames appear to be different, the hitches and most importantly the location of the axle. The axle on the European trailers is much more centered than ours here in the US. The tongue weights are much lower than the ones made for the US, apparently due to the prohibition of weight distribution hitches in Europe.
So, that said, I would be very careful when adding or changing the water tanks, both fresh, black and gray water. You will need to be especially concerned to not lighten the tongue weight, especially when the tanks are full. Tanks at the front add hitch weight, but again, it is unclear to me if the European hitch system is up to the addition of much weight to the front.
I was amazed at all the information in the manual on what we call sway here. They had different names for it, but it was obvious that it remains a problem with the European units. You must be very careful not to make that worse.
These are just general observations on my part. I doubt that anyone here has much experience with the trailer you have, and may not be able to help you much. You may have to be the pioneer on what you wish to do.