I certainly understand where you're coming from, and on these coaches in question I certainly would walk away - leaks are a bad deal.
As for the entire manufacturer as having issues, I believe thats painting with too broad of a brush.
How were these units set up? Were they properly leveled? Were the slides completely out?
I'll just use the Newmar as an example, although the way you have it worded I'm not sure if the slide was leaking or just the end cap. We've owned Newmars since 1999 and not once have we had a slide leak. Like all of ours, the Baystar still uses the original Newmar flat floor design. There is no new slide design on that Baystar. The new slide, "More Floor", the salesman mentioned is only on the new Dutch Star diesel and up.
I will say our Dutch Star gasser did once have a leak through the front cap, the front marker lights needed to be resealed. If I were guessing I'd say that was probably the case in the one you looked at.
On the Vista, this is the first I've seen of a leaking slide on that one. Again, in all fairness to the overall product, how was it set up?
I like your idea of shopping under those conditions, I just wouldn't scratch a builder when how the salesman set it up may be (probably is) the issue.
If a car saleman leaves a window down it's not Chevys fault it leaked.