I got told that allowing for a driver and a tank of fuel, the carrying capacity is an additional 1092 kg
We only use 650l tank, one set of treble ladders , an a frame, 2 pumps and reels and 3 poles and a bucket, and a few other bits
I'm having to go by memory Dave because the website where I used to check these things has changed format.
However, I believe that a 2.7 tonne Trafic had a tiny bit over 1 tonne payload (I think it was 1,012 kg but this may not be exact). The driver and passenger weight, fuel and equipment have to be deducted from this.
If you have been told that you can carry 1,092 kg OVER AND ABOVE the weight of fuel and driver, you may want to check this elsewhere too. Indeed, such a figure looks a little high even for a 2.9 tonner.
Say a driver weighs 90 kilos. The fuel tank holds another 90 litres (say 90 kilos). That's 180 kilos to come off the payload total if the diesel tank is full. I reckon that's about 830 kilos (variable depending on driver weight) you have to fit a tank, carry water and tools etc.
I once did a weighbridge check with mine. With all my tools and me in the van, I had room for about 770 litres. Everything else being equal, this would allow you about 570 litres of water. However, I am very overweight, my water system is heavier than some, and I was carrying more equipment than a lot of people do.
You may well get 650 litres in there but it could be borderline.
I suggest that you fill up with diesel, have an empty water tank, put in all the equipment you would normally expect to use and sit in the van while it is being weighed. Deduct the total from 2,700 kilos and that will give you the amount of water (in litres) that you may carry legally. If it's tight, you can always put less diesel in or only fill it when you will need less water than usual.
For the tenner it will cost, I reckon the peace of mind is worthwhile.