The vents to through the roof. I agree, on my FB once the grey fills it backs up into the shower. I suggest opening 1 valve at a time & turn on faucets to watch outside (assuming you have a clear section on your hose, if not I highlhy recommend doing so) to confirm they are draining. You shouldn't have to run much into each tank to tell for sure, so if they are in fact full.
Danny & Linda
Former '13 FB owner