According to the Health and safety Executive
here
legionella bacteria can be found in many natural water sources. below 20C they will be dormant and above 60C they die.
Most domestic water has been filtered and treated enough to reduce the risk of LD infection becoming a danger, but the problem usually arises in the way the water is stored or used within buildings, where tanks and pipes can be exposed to temperature's that can activate dormant bacterium and facilitate its reproduction.
There can be little doubt that water systems used in touring caravans can also experience ranges of temperature that would be suitable for the LD and other undesirable growths to multiply, especially whilst the caravan is in storage.
It is therefore a sensible precaution to thoroughly clean your water system after a period of non use, using sterilising fluids or other proprietary cleaners. Don't forget to also clean your shower head, and taps.
When in daily use, water systems will normally h ave sufficient through put to to prevent an unsafe build up of bacterium, also helped by the hot water reaching 60C in the tank.
If LD had been a big problem in touring caravans I think we would have seen reports of it, and a big push from the clubs and magazines about how to manage it, but I can't recall seeing any reports - suggesting it isn't major issue, however that doesn't mean we shouldn't take reasonable precautions and keep a watchful eye out for it.