What are you gaining by elevating your observing position ? Does it let you improve your horizon limits ? Most observing is done as near to straight up as possible. The lower you go the more air you are looking through and the more turbulence so most observing is normally above the 30 degree mark up to zenith.
Some one else pushed a water tank top build before. The comments if I remember correctly were about the rigidity and stabilty of the structure ( a 12" internal pillar helps ) but again also about thermal mass. Large concrete structures maintain their temperature for long periods especially if they are full of water, also a good thermal mass.
If it is just for a stable platform for visual and it is solid enough and gains you some viewing room over trees etc then give it a go. Chuck the tripod up there and see how it works for you. For testing it does not have to be permanent and it costs nothing to try.
Let us know how it works ..
Cheers
Brent