When we bought our caravan it had a pair of 59L tanks underneath that had been sitting there, in the yard at the caravan place, for months - with water in them.
For THAT job, I removed the tanks, used a 100% pool chlorine solution and shook the hell out of them. Then I used a 50/50 vinegar/water solution, and gave them another good shake. Lastly, after flushing them thoroughly, I used half a box of Bicarb of Soda in each tank, shook the hell out of them again, and rinsed them again.
Now, all I need to do is put about 2-3 spoonfuls (20ml spoons) in the tank during the last leg before we get home and then open the valves to empty the tanks when we arrive. Tanks are always perfect now!
The plastic taste might not be from your tank. We were fighting that plastic taste ourselves and couldn't fathom it until I tasted the water coming out of our garden hose. I compared it to the water coming directly out of the tap and whaddya know, the bloody hose caused it!
What's really dumb is I went to Bunnings and bought the best (and most expensive) food grade hose with brass couplings and took them home. I hooked those up and what happened? BLOODY PLASTIC TASTE FROM FOOD GRADE HOSE. I went and got my money back.
What I use now (exclusively, I will NOT use anything else) is a canvas hose on a reel. Cost me $125 or $130 (can't remember exactly) but I don't get any whiff of plastic. Water comes out of the tank in the kitchen absolutely flawlessly - because I also removed all the food-grade hosing in the van and replaced it with - wait for it - irrigation hose. Yep, that black shit they sell to set up your garden watering system. Not a single bit of plastic taste and the water tastes the same as it does coming out of the tap at home.
Did the job and not a noticeable taste.
