The vents for waste holding tanks protrude through the roof and should look something like this:
I took this picture a number of years ago (it's the black tank vent) because of all the flies that were accumulating around the vent. Once I emptied the black tank, the flies were gone.
Moral: No one can never tell me that my $#it doesn't stink! The picture is proof.
Now, about your leak. Is there a compartment door under your city water connection? If so, the water that you said was dripping outside from the water inlet connection could have dripped on and leaked through the compartment door or the caulking around the door. If you find wetness in the compartment, you might need to recaulk around the door and/or ensure that the door seals are water tight.
Don't even bother trying to make the waste tank capacity gauges properly. It's a losing battle unless you install something like a See Level system which will cost $$$. Checking the black tank's level is easy (but a little stinky). Turn the water off, open the toilet's blade and look down into the tank with a flashlight. About the only way you'll know the grey tank's full is when the water starts to back-up into the bathtub. In most cases, the grey water tank will fill before the black anyway. However, there are exceptions.
If you don't know your all your tank capacities, you might want to call Jayco's customer service and ask. I'm sure they can tell you.