Source for RV interior specs?

A few companies are better than others at providing this sort of info; Winnebago, as mentioned above, and I've seen some pretty detailed info from Airstream on their trailers.

In my opinion, it's probably not worth doing more than conceptual plans until you get an actual motorhome as some of the details are rather specific to the particular design/model. Some do have stuff hidden under the couch or dinette, some do not. Some have dollhouse-sized counters (as in the size you'd find in a dollhouse, not as in large enough to put a dollhouse on the counter), some have enough space to make a sandwich without having to cover the sink or stovetop.

Maybe you could plan some trips in detail instead; figure out your preferred route to Alaska, say, and research stops along the way....

Thanks, Ken! Those schematics are exactly what I was hoping to see. I wish all manufacturers were so forthcoming.
- Eli