Years ago we got caught and had to use a garden hose to fill our tanks. That plastic taste that is released from the garden hose stayed with use for several weeks.
Rule #1 is always use a food grade drinking water hose. Never a garden hose. Unfortunately, I have seen in the some RV manufacturing factories, when they test the plumbing system they fill from garden hose and in one case even from a fire hose.
This will taint the tanks, pumps and plumbing and used to be very hard to get rid. Until now.
An old guy came up to my stand at the Perth Caravan Show. Told me he had been in the industry for years and there is only one thing that will get the plastic taste out of the water tanks and that's - Cheap no frills red cordial!
I thought he was joking, but he was serious.
A short time later we hired a camper trailer. Worst water I have ever tasted, so I thought I'll try this.
I totally drained the tank. Poured the cordial in and fill it up. Pumped water through until it was coming out red from the tap.
I did leave it sit over night, don't know whether I needed to or not.
Next day, drained it again and flushed it out.
After re-filling, my kids drank the water- and they have bought up only filtered water.
So for a few bucks its worth ago, however, the problem could be the tubing that connects the tank to the tap. If its that clear tube, some of them break down after a few years and need to be replaced.
To test this, take some water from the tap. Then disconnect the tube at the tank, and catch some water in a cup. Taste test the tank water, if its ok then taste test the tap water. If its not ok then you know its the tubing.