The city water inlet does not go to the tank.  It is a hook-up for
pressurized water (from a hose).  I  never use this connection.  If
something failed, you would be flooded out in no time.  The key outlet is
for filling from the outside.  It is a pain in the ass.  If you go inside,
there is a large screw cap on the water tank.  (It is underneath the back
table, under the small flat top cabinets.)  Open it up and take a look
inside.  Easy to fill and rinse out with a hose ran in the side door.  I
only use the outside fill if I have to at the campsite.
The cap on the bottom of the sink drain is a standard garden hose thread.
Get a cap, or I use a hose connector that has the shut-off valve built into
it.  You can put a cap on the end of that for storage purposes.  The cap on
the water tank drain is larger, IIRC.
Live and learn.

Jay