For something like water/waste tanks, I'd think that relying on any information from another bus or a spec chart might lead to problems, other than for getting a general idea. Seems like there are so many variables in the individual builds on buses that a hands-on verification would be the best route to go. More often than not when discussing things like this, someone will offer measurements on something only to have someone else mention that theirs are totally different, or that they have such-and-such widget factory installed right in the middle of a bay which is wide open on another bus.