There’s like 24,000 uses of the key caravans. Although it looks to be confined to camp sites and camp pitches, but I don’t see why it couldn’t be used in other places. Outside of that there’s also caravan, but that’s for legal restrictions. So I’m not sure if it fits your use case or not. If you want too go really obscure there’s always capacity:caravans. It’s not used much and doesn’t have a Wiki article though. So probably it’s the least good of the three options. If it were me I’d just go with the first one, caravans.
AFAIK, not really, no. And if we had, people would likely complain loudly (like they do for e.g. class:bicycle) on subjectivity/verifiability basis, thus making it less likely data consumers would support it.
Yes… though they are also access method, i.e. “This property shows caravans are allowed on a campsite”. Thus adding caravans=* or caravan=* should only be done if there is sign saying it is allowed or disallowed (or perhaps if it is physically possible at all – although even that is disputed!) – and not to indicate how easy or hard it is.
Also, if it is not easy to use for for caravan=*, it is probably also not easy for hgv=* or bdouble=* (and a few more), so should those be tagged too? Or better tag maxlength=*, but that tag is about legally allowed stuff too, as those other access tags, so should not be abused for this.
For that however, the main question would be what app are you using, and is it capable of filtering by (or at least displaying differently on the map) that tag.
You could add a venerable (and likely supported) “description=hard to park with a trailer” tag to describe the issue, but note that this tag is humans only (so do not expect routing app to be able to filter automatically on it; instead you’d need to click on one by one to determine which would be good for you).
The advantage is, you can use that tag today, and it is somewhat usable by humans; but also if in the future some tag becomes usable for the “ease of parking for some vehicle class” purpose, it can be easily filled in by armchair mapper without resurveying.
And of course, you can micromap - instead of that POI being a point, map it as an area, with every service road, every curve, every width=*, every parking_space=* etc. represented.
That would described situation on the ground precisely, and with advent of smarter apps / AI it is quite feasible that relatively soon some apps might be able to analyze such data and give such suggestions automatically just based on such geometry. (And even if AI improvement were to lag, still “more detailed data = better” holds)
Having gone down and up the Adriatica countless times (A14), the service stations along the motorway often have entry signs guiding where caravans and motorhomes can find their parking spots and dump-station. To boot, height restrictors for the car park areas. as the operators would not want their costly solar panel covered parking roofs dislodged from their sockets. But for the occasional designated parking_space and dump stations to have mapped, kind of mostly done routing restrictions rather than the ‘friendly’ side. If an amenity is specific you might find some tagging along the said route. Milan-Bologna-Rimini-Bari-Brindise is the trail to follow.