That's a very touchy subject here. Since you are in Europe maybe you could actually help use and give use the tow ratings listed there for the 450 & 451. Some of us tow with our smarts. I don't do anything too big with mine but I would go bigger if I felt the need.
Most here have tent style caravans that are designed to go behind motorcycles but there are others who have small caravans that you can only lie down in.
Amazing how that works.... I keep thinking of merely taking my eons of experience and innovation and not wasting it here, but that would deprive the enlightened from partaking of it. Meanwhile, back at the ranch............... Jim Bob and Billy Ray are hard at work trying to convert their smart car into a CORN PICKER.
If you are a do-it-yourself type, a teardrop is a pretty easy thing to build. Any it's not particularly hard to keep the weight down. 300 Lb would be enough for a over-nighter, and 500 pounds if you don't need everything including the kitchen sink. There are plenty of light-weight builds that are holding up just fine over on the teardrop forum. The nice thing about a teardrop in particular, is that the shape adds to the stiffness and strength. Curved panels are much stiffer than flat ones. Many of the vintage trailers only used 1/8 skins under the thin sheet metal and they are still going after 50+ years. The aerodynamic loads are relatively low and, if kept light, the dynamic loads from the road are also low.
If I were doing a trailer for the Chevy truck I'd do a TTT (with Al skins -- they look so good), otherwise a small tear for the smart seems like it would be a good match (looks and functionally).
Yeah, The Falcon Sleeper is essentially an overnight sleeping pod and is kind of what I have in mind for a build. It is VERY lightweight, and would tow very easily and safely behind a smart fortwo. Ahhh Europe, where they have a concept of small.