yep they were the original optional equipment
It's the original equipment for the trailer. I know it probably does seem unlikely to try and put a full size heavy unit in a travel trailer, but these trailers, and my model specifically, were among the very largest ever built - 45 feet long, 8 feet wide so designed to tow on highways with no special permits/license or etc needed. It's 10 feet longer than the trailer used by Lucy and Desi in "The Long Long Trailer" (that was a 1954 Redman New Moon). And it is truly a travel trailer, not a mobile home. Yet it has a full size bath, with separate lavatory, large kitchen with full scale appliances, and so on. They are extremely strong, built with all aluminum frame and steel chassis. No wood or nails is used for framing. They tow like a dream. They were top of the line for the 1950s and when new they cost about twice as much as an Airstream or other competitors, and frankly they were worth it - At 54 years old mine tows like a dream, hasn't sagged in the slightest, and has never leaked. They generally came with the most innovative, top of the line appliances of the day, including the Duomatic. I love the Spartans because they really epitomized the advances of modern day conveniences, even while "camping" in their day.
I'm in no big hurry to find one as I am totally restoring the trailer to its original condition, but figured I might as well start looking now. I know of the Spendides and other new ones, but it would just not be the same. If anyone has one they are thinking of selling someday or knows of someone in my part of the world (Portland, OR) I would greatly appreciate the tip.