Every time we transfer water to our van, we introduce organisms into our IBC tank through the breather. They will be in the air that is drawn in to replace the water we've taken out.
Hence the reason why I commented earlier that this should be done daily as we did with our swimming pools. Dosing the pool daily was the only way to maintain it perfectly. Skip a day and we could see the difference in water quality, even if the pool wasn't used.
From what I've read, legionella is in pretty much all tepid water but takes time to breed to create a problem. The data around the web is very vague but all suggests having a routine to get back to zero occasionally (which is almost how vague it is). My IBC is black and in full sun so I deal with it occasionally, hopefully enough.
Vin
This year is the first year where my water is tepid in the van tank and we are in the North East.
The first system I had was a trailer with the tanks exposed to the sun. We had to drain them and clean the green out of them every couple of weeks. But the water was never warm like it has been this year.