Outfit: Toyota Hiace "camping-van"
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.."IF" the switch in incorporated in the main control panel, then as Jeff says, it's probably the water pump..
(..but that would've been easy enough to fathom that out,with a bit of button pushing..so it maybe it isn't that then..?)
As you/he hasn't got any handbooks, it would be worth contacting Burstner and seeing if they could supply something? http://www.burstner.co.uk/
(..or if bought from a dealer, chase them up and get them to source some )
It may also worth just having a search online, to see if there are any PDFs available..?