I'm typing this from the inside of my 2003 Cougar 278! My wife and I have been living in it full time since March. I don't think you can get much better for the age and size. The rear kitchen is huge compared to similar trailers of the time.
Here's a video of the inside of mine if anyone wants a better idea of the layout.
Your plan for the bathroom sounds interesting, can't wait to see how it turns out. Any chance you could post a couple pics of the belly pan? Mine doesn't have one and it would be nice to see how it was implemented from the factory. Is that an outside shower just ahead of the slide? Mine doesn't have one but there are signs that something could be put there.