New plastic water tank 'taste' ??

There are lots of recommended fixes for this,

I did this and it seems to work Dissolve about 500grams of BiCarb Soda in a bucket of warm water and pour it into the tank, then fill it right up. Leave it soak overnight then drain and rinse and refill.

Others recommend a bit of household bleach and do the same as above.

One thing to make sure of it that you only use food grade hose to fill the tank and as a supply hose for the van.

As well as Notso's suggestion, it may not be the tank.
#1 don't use a garden hose to fill the tank - might be the source of the plastic taste. Best hose I've found is the braided light blue drinking water hose made by Nylex or Neta. I had bad plastic taste in the past from Pope brand drinking water hose.
#2 The hoses used to plumb your water tank might also be the cause. If the first glass of water is bad and after running more from the tank, its OK, then blame the hosing.

Milford's baby bottle wash is what I use.
Comes in a 1 litre plastic container and from memory, the measure is 50ml per 1l of water.

In your case, add half a bottle to the tank and fill it up and leave it for a couple of hours or so.
Then turn on the outlet tap to flush a bit through the plastic tubes, then drain the lot.
Refill with clean water, flush the outlet tube, drain again then fill the tank with clean water for the final time.

I also use a water grade hose to fill the tank of my camper as cheap garden hoses will just taint the water taste again.