What it is, is that some people are simply more sensitive to the taste a typical RV water hose imparts to the water from the vinyl it's usually made of. If anyone has ever tasted tea from a styrofoam cup, you'll understand how bad taste can be from the plastic molecules it picks up.
Valterra makes a "taste free" hose. Doesn't say what material it is but is says it's made from an inert material and not vinyl.
Flex-Tech makes an RV water hose made from PVC and are available in 5/8" dia. (better flow) and are rated at 150 psi (higher than Valterra). Whatever hose you get, it needs to be listed for potable water only.