When the mfg.'s drill holes in plastic water tanks to install fittings and the shavings fall into the tank and stay there for a free ride to their new rv homeowner...you.
Sometimes those shavings do not remain in the bottom of the tank , they become bored and start to travel to the filter of the water pump or somewhere down the water line. They stop traveling when they become blocked by a valve or whatever.
It seems to becoming more common for a low point drain to drain the line as well the FW tank. As suggested, sometimes you can work a valve a few times to allow it to work sometimes you need to cut and replace.