>Assuming that this is an American model, a regular hose cap should fit 
>perfectly, as long as it is the right diameter...  I know this only because 
>mine was unscrewed during shipment and fell off during delivery.  I discovered 
>that in order to replace it, at that time, I would have had to replace the 
>whole sink drain assembly (really stupid situation) in order to get just that 
>plug.  I went down to the local H/W store and got a cap that fit perfectly and 
>voile'...

I don't think that the original poster was thinking about the sink drain...
I believe he was looking for a water tank cap.  For the sink drain, I've
heard that the cap from a Chlorox bottle will work.  (I haven't tried it,
though.)

Ron
--                                       
Ron 'Coyote' Lussier         /\_/\      ____             Chaco Communications
(408) 865-0657              (     )     \ _/__        10164 Parkwood Drive #8
coyote@chaco.com             \   /       \X  /       Cupertino, CA 95014-1533
(1990 VW Westfalia Camper)    \_/          \/    http://www.chaco.com/~coyote