It sounds like hydrogen sulphide which is caused by the microbial breakdown of organic matter in the absence of oxygen. The grey water tank is the most likely source of this smell, are you sure its from the tap and not the plug hole?
We occasionally get a similar smell in our bus and have tracked it down to air from the grey water system coming back up through the drain(s)
For piece of mind I'd also give the fresh water tank a bloody good clean out and see if that fixes it. You may have picked up some less than pristine water from one of your fill ups.
1993 7m Hino, 500w Solar, 572ah AGM, 2kw PSW Inverter, Gas hot water, 180L clean & grey water, 12v 133L Vitrifrigo fridge, 2KW Webasto diesel heater. Kings Quest auto sat dish
Practising for retirement