I have a 03 Montana 5th and I dont see how the Galley could be leaking at the vent. Did they mean the Black tank? The sink would back up first. I was at the beach a few weeks back. The campground set up a camper accross from me. They hooked the hose to the sprays input for the black tank by mistake and it came out the vent on the roof. They had to wash it. Mine has three tanks and yes the shower can fill up quick. I normally leave it open until the day before ready to leave. Make sure you have the hose hooked to the right inout.
2003 38' Montana 3S, 2006 Dodge Ram Dually 5.9 4WD, Auto, Derale DD Tranny Pan, Ispro EV-2 Gauges, FASS Fuel Pump.