Close them! You never really get out EVERYTHING that is in your holding tanks, so you don't want anything extra coming out when you are not hooked up to a dump. Also, the tanks are vented through pipes coming out atop your roof.
My personal opinion, and way of tank management, is to purposely keep SOME water in my tanks at all times WITH a probiotic/enzyme type tank treatment such as TankTechsRX or Happy Campers. This way, even when the RV is sitting, the yucky is being taken eaten up by the micro critters.
I also believe that the first modification that should be made to any RV including brand new ones, is to install the Siphon 360 holding tank vent covers, generation 2 for aftermarket installation. Yes, they make that much of a difference.
Best to you.
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2005 22' BT Cruiser 5211, Chevrolet 3500 chassis w/6.0L Vortec V-8; 6'x12' Cargo Trailer; Honda CB500X motorcycle for street and ADV riding; Yamaha TW200 motorcycle for dual-sport and trail riding...All of it for fun and adventure!
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