Hi Steve
You can use the products mentioned above but you will get and after taste from many of them and they will only do half the job.
If you use Kemdirect cleaner / descaler you will remove both the germs and bacteria and the calcium build up ...if any.... where many of the germs live.
With some of the products suggested you will not remove scale as they are not descalers only cleaners.
Kemdirect is completely biodegradable and leaves no taste after rinsing the pipes or tanks etc.
You can use it safetly throughout the caravan both in the drinking water and waste water and then take it into the house and use it as a pure descaler in kettles, shower heads etc. Here you can actually see it working.
It is available from www.trail-a-mate.com as mail order or through some dealers and suppliers.
Hope you find a product that you feel safe with and does the job.
Joyce