Our rig fills with sewer smells and there’s a vacuum when dumping. Why?

Dear Dave,
Recently our RV literally fills the RV with a sewer smell when I empty the black tank. We are set up in a camp for 4 months, so I drain about 50% of the black tank once per week. I’ve noticed lately that when I close the black tank valve during or after dumping, it’s pulling a suction. Seems like a lot of suction, in fact. I don’t ever remember that happening before. It’s almost like I have a vent plugged. Does it vent to the roof, I guess? Thanks. —Kelly, 2011 Palomino Sabre 5th wheel

Dear Kelly,
According to the specifications I found, your Sabre has one black water holding tank with a capacity of 42 gallons. You did not state which floorplan you have. However, there were 10 models ranging from the 28 RET to the 32 REDS and each had 42-gallon capacity.

It also looks like all the floorplans had the bathroom midway to the front with either a full bedroom in the front or a jiffy bed and bunks in the 32QBDS. This most likely means the black water tank was positioned in that general vicinity underneath with a remote pull handle. From the walkaround videos, I see most had the dump valves exposed on the driver side of the rig just forward of the slide room.

I was not able to see any footage or photos of the roof. However, I would assume that in 2011 Forest River would have been using the traditional sewer vent pipe that is a 2” PVC pipe going up to the roof and a flat cap like this sample photo.

All sanitary systems have a ventilation system

All sanitary systems in both residential and RV configurations have a ventilation system that reduces the vacuum you would get when dumping the tank or flushing a toilet in a house. The pipe either comes off the tank itself or is plumbed into a drain line. Most black water tank vent lines come directly off the black water tank and run up through a wall in the bathroom to the ceiling.

Here is a 3D diagram from a 2011 Winnebago Adventurer that shows the black and gray holding tank vents.

The vent may be plugged

I would believe your floorplan has a similar design with the pipe going up to the roof. If you are getting sewer smells inside the rig and a suction effect when dumping, I would suspect the vent has gotten plugged, as you suspect. The first thing I would do is inspect the roof and most likely install a new vent cap. Most RV manufacturers use the very inexpensive cover cap and, depending on the wind direction, may not be very effective.

I would recommend the Siphon 360 to replace it. It has a built-in bug screen, is a universal fit, and is designed to remove odors more effectively. You can find it on Amazon here.

This will give you the opportunity to inspect the pipe coming up through the roof. If it is loose, it most likely is not connected to the black tank anymore. I have come across this a few times where the pipe connection lets loose or breaks at the tank or the bend. This would account for the smells, but not so much the suction or vacuum, as an open hole in the tank or connection would still create an open air source for venting. So it might be the pipe broke and collapsed or pinched just enough to allow some air in but not enough. If it was completely plugged you would notice a pulsing or surging in the dump activity.

Do you have a “cheater vent”?

If the black water tank is located in a position that does not allow a full length pipe to get to the roof, such as the situation with a counter or other open floorplan, RV manufacturers run the pipe up through the floor and lower cabinet and place a siphon cap, or what we call a “cheater vent.”

This is designed to allow air to be drawn in when dumping the tanks but also have a one-way rubber gasket that keeps odors from coming into the rig. They almost all fail, and if the gasket collapsed, that would cause the vacuum, as well. Check under your bathroom cabinets as well as the kitchen cabinets. We found one hidden behind a corner cabinet in a 2015 Raptor Toy Hauler and had to take the front panel off to get to it.

If you’re in the area, be sure to attend Dave’s seminars at the New Hampshire RV Show in Bedford, today through Sunday. You’ll be educated and entertained, guaranteed!

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Black tank still smells after trying EVERYTHING

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Read Dave’s answer.

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Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

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