I used Milton baby bottle cleaner on the tanks in my caravan, with good results in that no one was ever poisoned. The stuff is available from all the super markets.
I brought my tanks back from properly horrible, the heads tank was out of service for over 9 years and was ugh, the galley tank also laid up but brought back as it tasted icky.
I used puriclean from a caravan shop, I did not have much luck with their recommended doses to try to save the galley tank and as a last resort decided to 'shock' it. I poured half a tub in and left it fizzing for a few hours. This has worked a treat, I have now had clean tasting water for 5 years (was it that long ago?) and in the heads 3 years.
We use Milton baby bottle cleaner bought in our local supermarket. I have just cleaned ours and flushed through to remove the taste of the Milton. (Sold under different brand names around Europe but basically any baby bottle steriliser fluid will do.)
We have 3 tanks that hold 270, 160, and 160 ltrs of water. What we do is:
Empty the tanks.
Pour a bottle of Milton into the filler
Fill the tanks 1/3 up
Pour in a second bottle fill up another 1/3.
Use a third bottle fill completely up.
Whilst filling up the last 3rd I pull some water through the HOT water system, cold water system and manual pump.
Then I leave it all sit for an hour or so then drain all the tanks. This gets them clean but still leaves some Milton in the subsystem so I re-fill with just water and drain again to remove any Milton taste form the water. You do not need to do this last step as the Milton is perfectly OK to drink but makes tea taste a bit horrible so i choose to flush it out of the system.
Alternatively their are all sorts of proprietary cleaners available most of which seem to have the same ingredients as Milton but are much more expensive because they are bought form chanlders and not the baby section of your local supermarket.
In fact it's not actually necessary to fill a tank to sterilise it unless you have 'scum' clinging to upper surfaces. The sterilising agents fumes are themselves good enough to sterilise. I only know this from Beer Making - as sterilising agents supplied for fermenting bins explain it all.
Biggest problem for me out here is no Baby Bottle fluids available. So have to use plain supermarket bleach. But this does have one advantage - it's strong !! and also fumes really do a job !
Milton worked well for me, and after a few refills the taste is very good; I have no hesitation in using the electric pump to empty 600 litres. Do clean the filters too.