On mine, the serial # is embossed into the structure which you can see inside on top of the rear deck area near the hatch opening.
I'm going to have to look for that on mine this weekend, haven't noticed it yet!
I'd also be psyched to see what you've got going for mattress tie downs.
I've been working my own Franken-Nemo bed set up and I'll post photos of it after this weekend's shake down if all goes well.

That said I'm still trying improve on keeping it up top during closing.
Might be time to start a Habitat Owners thread separate from the Official Release thread.