It won't be as far back as the bedroom - the water tank is likely closer to midships and near where your city water hook up. More likely to be on the driver side too, though I've heard of a few on the passenger side. Even owned one like that several years ago.
Some rigs don't have a gravity fill and use the city water inlet exclusively. Don't worry - there will be a separate vent opening somewhere - you should not have to remove the gravity fill cap to fill via the city inlet. Just find the lever that changes from city in to tank fill and wait for it to fill.
Hopefully another Winnie owner will know exactly where yours is. What year and model Winnie is it? That's important info to know when you ask a tech question about your rig.