I dig prefabs as well. Three in an RV would get me thinking about a small abode as well. My fiancee actually loves looking at smaller homes a lot, and is quite keen on them, but she is a MASSIVE thrifter and collector. No matter how much she would want to move into a small footprint house, I think it truly goes against her nature :) Our place now is less than 900 square feet. Most people in the suburbs would say that is far too small if you have a kid, but not me. I personally hate houses that are so large as to have unused or non-purposed rooms, and our place has a basement which can be finished to double our capacity if need be. I'll probably build an office down there in a couple years. The only reason I'd consider getting a different place is for aesthetic reasons, -a MCM ranch would just be pretty hard for us to pass up.
My wife shares your love for Mid-century ranch style and tiny houses.
Weird is there some common trait in the styles or is it just design fetishism. Both deal with spaces with multiple uses.
I think it has something to do with deliberate design with purpose. MCM was actually a very strong purpose driven design movement with a hard perspective. The same can definitely be said of the prefab movement and things like shipping containers as homes. Both place a premium on both functional design and aesthetic (at least in the majority of the lustful prefab homes I've seen), even if the scale is different and the objectives of the design are different. Both have kind of a 'top down' design philosophy where everything flows from purpose.