You can find out if you have a one vent system just by looking for them on your roof. Mine is directly above the wet bay compartment.
The funny thing is I am not sure if your toilet can fill the black tank and itself in a short time...if your foot pedal valve is not shutting off, it would fill the bowl, if the flush valve in the bowl is not closing and therefore leaking water into the tank the bowl is not going to fill up, well it might but it would take a long while if your coach is plumbed like mine. I suggest you drop a toilet packet in the bowl, let it dissolve which will add color to the bowl water and then see if it fades due to leaking into the tank.
Oh, buy the way, I had a leak between the city water and the fresh water tank. I shut the water off and rapidly worked the lever a few times, the leak stopped. I suspect the ball value may have been stuck in a slightly open position and I exercised it loose.