I was told they never installed toilets originally--but SteveV, you prove me wrong.
A Thedford "recirculating toilet" is still being sold
and can be seen online...this was designed, apparently, to be used with just a "slinky" or with a portable holding tank and DID have the hole in the floor. No wonder so many have that hole. It WAS fron the manufacturer.
From the Thetford Corporation website:
Electra Magic Toilets - Thetford Corporation
Find a Dealer
For use in boats, RVs, buses and other land-based and marine applications
uses very little water Ideal for those who don't have a holding tank
Convenient push-button flushing
Operates on 12- or 24-volt hookup
Large, contoured seat offers full-size comfort
Easy-to-read Level Indicator
Caution: Do not use stiff brushes or homemade products to clean or lubricate the valve and seal, as these will damage the seals and surfaces, causing odors to escape
from holding tanks. Thetford recommends using Aqua-Clean and toilet seal lubricants.
Product SKUs: 24345, 34344
There's a whole PDF brochure you can look at; it even shows the hole plan, and it explains some of the layout of the rest of the trailer...we found it very elegant already, and this makes me admire it more. So much planning went into these!