My RV bathroom has sewer flies even with tank valves closed. Help!

Dear Dave,
The RV’s gray and black tank valves are closed, so how am I getting black sewer flies in my bathroom? How do I get rid of them? —Laurel, 2006 Holiday Rambler

Dear Laurel,
Even with the tank valves closed, sewer flies can still cling to the sides and top of the tank and drains and hatch larvae and more flies. Rotten food and organic matter are breeding grounds for these. They typically lay eggs in the slime buildup on the pipes and even the drain strainer.

Their lifecycle can be as little as one week. However, typically it is 2-3 weeks and each adult female can lay between 30-100 eggs, so it is difficult to stay ahead of them if you don’t treat your tanks periodically.

I’ve had people question how they can get flies inside the rig when they have not opened the toilet. Sometimes a sink in the rig is routed to the black water tank due to the floorplan design. As you travel down the road, the swaying can “slosh” the drain water out of a “P” trap, so there is an open path until you run water down that sink.

It is also important to use the proper treatment that is enzyme-based, as the enzyme will help break down the slime and kill the larvae, as well.

Why do you have flies?

Most likely, the flies are coming from some type of food source that is getting washed down the drain or the toilet. It doesn’t take much when one fly can lay as many as 100 eggs, and they are adults within a week!

And it might not be your tanks, as flies can breed in waste foods in the trash, leaks under the sink, especially a “P” trap leaking, the basement, and even the sewer hose if not properly cleaned and sanitized.

Getting rid of sewer flies

Dump your black and gray water tank as usual. Bleach is my sanitizer of choice. I use 1/4 cup in a gallon of water for many things, especially sanitizing the tanks periodically. I also put it in a spray bottle and squirt the dump valves, clean around the toilet (if I don’t have carpet!), spray my outside faucet and shower, and even the campground faucet. You never know what has been there.

In both the black and gray water tank I use one gallon of the bleach mixture for every 15 gallons of capacity, and then fill the tank to the top as larvae and eggs can be there, as well. Let it sit overnight and even drive around the block to get it all over the tank. I like to pour the bleach solution down all drains, toilet, shower, and sinks. Caution: Do not mix bleach with anything else as it might create harmful vapors. You can also use boiling water or drain cleaner to clean the pipes.

Then dump the tanks at an approved dump station and rinse them out two more times. If you have a black water flush valve, that is good to use between rinses. Or you might be able to use a flush wand.

If you don’t want to use bleach, then use an enzyme-based cleaner like Thetford Tank Blaster or a residential one that is approved for plastic pipes.

Use an antiseptic towel to clean all the drain pipe connections, under the sink, the toilet and base, and around the shower basin. You will also want to clean the dump valve and surrounding pipes and compartments. Most of the service compartments are plastic and can be sprayed with the bleach solution and rinsed off after an hour. Just make sure you don’t spray any electronics or wiring. And if you have a washing machine, don’t forget to clean and sanitize that and the surrounding area as well!

Getting rid of the adult flies

After sanitizing, you probably don’t want to wait 2-3 weeks for the existing flies to die, so you can do a few things to get rid of them. There are several DIY traps such as apple cider vinegar and dish soap covered with plastic wrap and a few holes poked in the plastic. Or use a commercial trap available at home improvement stores and hardware locations.

Quick tips for keeping them out

• Clean and sanitize both the black and gray water tanks often.
• Don’t leave dump valves open.
• Keep your drain pipes and “P” traps clean and sanitized with bleach solution or boiling water.
• Don’t let food sit in under-sink trash cans.
• Drain tanks and sanitize before storing.

 You might also enjoy this from Dave 

How do I keep my RV’s black and gray tanks clean?

Dear Dave, 
Good morning. Other than the Thetford gauge cleaner, what is a good routine to get into to keep black and gray tanks clean and not smelly? Thanks. —Julie

Read Dave’s answer.

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

Read more from Dave here. 

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