Fresh water holding tank vent will be near/within the fill port.
Waste water holding tanks will vent up through the walls and out the roof. There are plastic vent covers on them. They don't open or close more than they already are. Yours maybe broken though, because they are plastic and do get brittle. It's possible a bird or bees nested in there and plugged it. If you take the cap off and don't see a problem, you could try putting a garden hose down it. I'd personally try to pressurize with air first though. It's just a straight 1" or 1.5" PVC pipe that goes from the top of the tank up to the roof.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR