Rainwater cycling and taste question

Hi folks, we have our rainwater catchment setup and collecting rain at a rate that's way higher than we ever imagined, which is phenomenally cool.
Our current setup is metal roof > PVC eavestrough and downspouts > food-grade rain barrel (ex maple syrup) > 40 gallon first flush > 4 x 1000L IBC totes > Sediment filter > faucets etc.
I have two questions.
Firstly, we currently have all 4 totes feeding into our system at the same time. Water enters 2 of the totes and then it self levels across all 4. Should we be cycling the totes to prevent holding water in any single tote for any length of time? I was thinking of closing one or two totes off from the system and emptying the others before turning the empty ones off and then feeding off the full ones. Would this be a good idea tp pursue or should we just let the levels "bounce" across all four totes?
Secondly, the water has an ever so slight "taste" to it, my g/f uses the words "oily" and "gasoline" although I wouldn't describe it that way, however, there is definitely a taste, not super strong but it's there. We sanitized the totes and the rain-barrels with baking soda before use but I'm wondering if there isn't some kind of coating on the roof that might be behind this. We tried putting some special "sock" style filters on our downspouts but the water just backed up way too far for this to be an option. Any suggestions?
We do have an RO unit that we will most likely be installing once we get our kitchen built, would that resolve the issue?

Our rainwater cistern is a stainless steel milk truck tank. The roof and gutters are galvanized steel. The underground water pipe is poly, and there is a carbon filter block at the house. You can read about our system here: http://geopathfinder.com/Rainwater-Harvesting.html
Maybe the PVC gutters and plastic tanks are off-gassing? Plus plastic can get scum growing on it over time. We only use plastic tanks for garden irrigation. Is all of your plastic food-grade? It seems the more likely source of funky taste as the water is in contact with it for a longer time than the water that runs off the roof. Also, the time of year and pre-rinsing of the roof are important factors, as are tree buds in spring and leaf residue in fall. For tank sanitization I would recommend hydrogen peroxide, the commercial strength 35% if you can get it. Used in milk processing plants around here.

I have a lot of experience with rainwater collection. I have a big topic somewhere on Permies.
I think you IBC's will settle down, there is usually no issue with them if they are food grade.
Sometimes when people taste rainwater they think its no good, because it is different.
PVC gutters do not normally cause an issue.
You will not need to wash the roof etc, since the first flush is dealing with that.
Your first flush water volume may be much more than you need, study it perhaps.
keep all tanks connected, water does not go off sitting in tanks, and the greater volume you can catch and store is always a good thing.
I have never seen algea grow in these tanks, I store about 300,000 Litres of water in a range of plastic tanks, which is common in Australia.
What is an RO unit please?

Reverse Osmosis unit for water treatment.
I can say that in Australia I know of not a single household that treats it water captured from the roof.
Now things may be different in North America, acid rain, industrial pollution, but I actually doubt that is an issue.
I have often heard of people expressing concerns about rainwater and I never understand where the fear comes from.
I am a humble Civil Engineer, who has worked with many issues, including rainfall collection over many years and I am still puzzled by the comments about the nee for treatment.
If somebody can clearly explain why it is not o I am keen to learn about it.
Thank you.
And good luck with your water, buy another tank rather than an RO unit perhaps.