The following is the contents of a PM from Brian who answered the original question. Since others have the same questions I am releasing this to the public.
I think that I found the fresh and grey waste tanks to be the same size - about 100 L. The black tank is much smaller.
Thanks Brian, How did you find out? I have been wondering about the weight
. 100L is 100Kg or about 225# plus if you fill the hotwater tank you will get another 20L.
I basically just multiplied the length by the width by the height. From memory, the tanks (which are openly accessible from below) are about 20 cm x 60 cm x 100 cm, or 2x6x10=120 L, less some due to the sloped bottom and shaped sides.
Someone did find a supplier of these tanks: DM Plastics
Canadian source for water tank for 17 ft. Boler
It looks like my estimate was about 12% high.