Although I agree in principle with what Henry suggests, in thinking about why RV manufacturers don't simply use radiator hose in the first place I surmise that the interior lining may not be nearly as smooth as Flex PVC, and over time friction may result in some degree of debris buildup clinging inside radiator hose that's designed for water and antifreeze mix only. I'm sure some consideration goes to the theory that rubber eventually will break down, whether it takes 5 years or 25, and PVC won't. Except of course where it gets overstressed due to improper installation techniques by coach manufacturers. The Flex also fits inside fittings and is supposed to be set with PVC cement/glue; the transition between fittings is relatively smooth like drain systems are designed to be - no fitting rim jutting into an outflow to trap or slow it. Unless you used a radiator hose with at least one flared end, the outflow end would violate that drain plumbing principle because instead of being glued inside that outbound fitting, it is clamped around its outside, so the fitting's lip inside stands against the flow.
All that said, admittedly there are places in home drain plumbing where rubber fittings and conduits are used to provide flexible adaptive, hose-clamped connections between piping, such as disposals, dishwasher connections, and ABS-to-iron sewer pipe adaptions. They aren't subject to the same substantial heating and cooling of pressurized liquids within them that automotive radiator hoses are.
Just my 2 cents and welcoming anyone else's.
-Joel
