Thanks everyone for the information.
DPeakMD, yes some of the rear floor under the the bath counter has been replaced and is appears I may have the sagging tail syndrome, I am investigating that now.
azflycaster, I replaced the furnace last week, you can see the thread here: http://www.airforums.com/forums/f427...4sp-47264.html
the piece of duct work that is missing was replaced by the PO with some flashing, I'll have to fabricate some new duct with a hose running to the fresh water tank for heat.
I did figure out where the ducting is to the grey tank, see photo.
Also here is a close up photo of the hole I am wondering about that is by the shower pan.