The pictures I attach here show my 25 litre / 6.6 US gallon water container in the travel and the in-use positions.
Taken during different trips.
It works well. There is just enough depth in my galley to allow me to just tilt it over and stand it up again, with no need to slide the whole thing in or out.
As a person who has had to drink some less than pristine water from an old tank at sea, let me assure you it is also a great advantage to be able to drag that tank out to discard the stale water and give it a rinse occasionally, rather than constantly refilling a built in tank.
25 litres is 25 kg or 55 pounds, but it is tucked in behind the axle. The ratio of the distance from the tank to the axle, and from the axle to the hitch is at least 6 to one on my trailer.
So the difference between a full tank and an empty tank on the towball downforce is around 4 kg / 10 lbs.
Things are tough if that's all it takes to significantly affect the handling under tow.

- Park.JPG (158 KiB) Viewed 1308 times

- Ready to leave NYD.JPG (91.78 KiB) Viewed 1308 times