I might ask whether there is possibly a "P" trap somewhere that may have dried to the point there is no water in it....possibly a washing machine that hasn't been used recently.
There might be a "gas vent" beneath one of your cabinets that could be stuck open also.
....Just a couple of things you might check for.
Not telling you what to do, but a grey water tank can develop an incredibly unpleasant odor what with bits of food scraps, etc getting washed down the sink....not to mention a full tank simply empties better and carries that debris with it.
On our MH, we hold both tanks. When I dump, the black is first with the grey next for the first rinse of the hose, etc. If you leave your grey valve open there's a straight shot from you nose to the gopher hole, if you do happen to have a gas/smell leak somewhere.
