Shasta SC black holding tank

I might be wrong, someone please correct Me if I am.
With the cut off valve that close to the floor, it may not have had a black water tank.
I think the valve, when there is a tank, is located on the outlet side of the tank.

Considered a park model when no black holding tanks. Your dump valve is right there at the exit point of the bottom of the toilet so if there is any holding tank at all, it's above that. And so far I've found that it is not easy to add  one to a trailer that was not designed for it. Remember you need a certain amount of clearance under the rear portion of the trailer for those steep driveways and such.

You can add a brand new toilet that has it's own little holding tank and it can be dumped the same way as the one you have now through that pipe and valve except it will have it's own valve on the bottom as well. Thetford makes toilets like that. I installed one in the 59 Kenskill using the same through-floor plumbing so you just hook up a hose and dump it like normal. 

I used the existing flange and everything for this toilet install. There was already water there so I added a hand held sprayer for cleaning purposes and to fill the toilet tank with water for flushing. You can see the dump handle at the bottom.

This is a photo of the tank removed from the trailer, out of a 1961 Astrodome.  The Astrodome was the same trailer as the 16SC but had the front bunk overhanging the hitch.  I don't think there was much structural difference form '61 to '64. 

(Thanks go to schweetcruisers , again, for this photo lifted from his blog:    schweetcruisers.blogspot.com/   )

Since we are having a conversation about weird vintage toilet configurations, I am wondering if anyone here has actually had one of these pumps in their trailer? As far as I can figure out, it is a basic hand pump that brings in water from the fresh water tank. I am assuming it was the only way to flush the original tank as my trailer did not have an electric pump. My new replacement toilet is a direct flush china bowl that Vikx recommended. We will add direct pressure city water hook up to the trailer, but I am thinking of keeping the old pump for when we do not have pressured water. Anyone have experience with one of these??

Can you explain what “siphoning” back means? I’m think what you saying is that I would need a back flow valve on the line, to make sure the water only good one way? Is that correct?

Yep.