Before jumping imediately to the conclusion its a fault inlet on the side of the caravan, A little more information might enable us to pin point the problem(s) more accurately.
If he has a pressure switched system, then it points to a faulty presure switch which has an NRV built into it.
Its more likely he has a switched tap system.
When he turns his cold water taps on, does he get instant water or does he have to wait for it to come through?
If he has to wait then it indicates two problems. One would be the non return valve in the inlet housing (and at 20 years its most likley a Carver unit) But also it means that one or more cold taps is not sealing preoppely when its turned off, as its allowing air back into the pipework taking up the space the receding water occupied.
If he is only having to wait for hot water flow, then it points to the non return valve on the water heater having failed and possibly the inlet NRV also (though some systems had a seperate feed for the water heater that was not protected by the inlets NRV.
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