Some people are apparently sensitive to taste due to the material a hose is made from and some aren't.
If you are having an issue with taste, it's not going to be the from the PEX water piping or tank which are made from food grade polyethylene, but could be from the hose used to fill the holding tank.
Not sure exactly what a taste-free hose is made from but still has vinyl in them. Hoses that aren't rated for potable water may have nasty chemicals and metals in them like phthalates, BPA, antimony, lead and flame retardants. Only RV hose I will use is NSF certified for potable water but doesn't need to be "taste-free" for us.