The surest way to tell if your membrane (RO) needs replacement is by the sudden over production of "purified" water. I have had one unit fail in ten or so years, multiple applications. When the membrane fails, it blows out and you no longer are forcing water thru the unit, hence you will see a volume increase to what other faucets are producing.
My unit in the bus has three pre filters, two sediment and one charcoal....they are the ones that hang on the rack. After passing through those filters water is then directed to the RO membrane and finally finishes with an additional filter prior to going into the accumulator tank. The air side of that tank is critical, too little pressure and the water does not want to come out, too much pressure and the water does not want to go in....this is another item that Watts advises checking on annually, just a schrader valve. While these systems are very simple, there are a couple things that must be done with some degree of regularity to keep them operating......pre filters (and I suspect that yearly might be a bit of an overkill for a bus that is not used full time), the pressure in the accumulator as I have noticed mine tend to flatten out, or lose pressure.
I think most of these coaches have a water pressure regulator built into the system where external water connects, if not, probably would not be a bad idea to install one.