Poly tanks usually don’t have a plastic taste after an initial fill and flush.
Any plastic taste usually comes from garden hoses or poor quality food grade hoses used to fill the tank.
To clean the onboard water tank, try draining the tank, then refill with fresh water and a cup of (Non Scented) Bleach. Any brand that you use at home will do.
Leave for a day or two then empty the onboard water tank.
Fill & flush, then refill again.
This is the most common method and usually works well.