Its actually the law to have "do not overtake turning vehicle" signs on heavy vehicles over 7.5 meters long, articulated or rigid, this is a warning to car drivers NOT to overtake when a truck is turning (which is illegal) as far as caravans go, the combination would exceed that length on most set ups however they don't need anywhere as much road when cornering, much less likely to 2 lanes when cornering
As far as "long vehicle" signs go, 1 trailer semi's can operate up to 19 meters, no signs required, sometimes you see what truckies refer to as pocket B doubles, they have 2 short bogie axled trailers (often fuel tankers) also come in just under 19 meters, again no signs required.
Anything over 19 meters, such as earthmoving floats, extending trailers, single trailers towing a roadtrain dolly, full sized B doubles, must carry long vehicle signs.
Oversize signs are for stuff that is WIDER than 2.5 meters regardless of length and have to have flashing amber lights and some cases permits.
I personally think that if any outfit is over 19 meters long (that's a bloody big caravan), then it should carry long vehicle signs,
Shane