Full Hookup Disasters – What Most DON’T know!

Full Hook-up Disaster!

So, you’ve been camping for 50 years now and feeling that expert mode, right? Or perhaps you’re a newbie and aren’t quite clear on how holding tanks work? Well there’s a problem with the way you hookup and use your sewer connection and you probably haven’t even heard about it! It’s the dreaded sewer or drain fly!

Common Drain Fly
Common Drain Fly

The Psychodidae, AKA Drain flies, sink flies, filter flies, or sewer gnats are small true flies with short, hairy bodies and wings giving them a “furry” moth-like appearance, hence one of their common names, moth flies. There are more than 2600 described species worldwide, most of them native to humid areas, like Florida, Georgia, Alabama, Louisiana and Texas coastal areas, however they can appear almost anywhere in the summertime!

So what does this have to do with camping?? I’ll tell you! Do a simple Google search for RV sewer flies and you’ll get about ONE MILLION results!  But why so many results?…

Because you are USING YOUR HOLDING TANKS IMPROPERLY at the full hook-up site!



If you currently have drain flies, this is one of the best natural products to clean your tanks with. Cleaning instructions are at the bottom of this article.

Dangers of Misusing the Holding Tanks

Posted on the Internet:
I just purchased my first ever motorhome earlier this year, a used 38-foot Damon. I have what seems like fruit flies breeding in the black water holding tank. No problem with the gray water holding tank. Whenever you push the flush foot pedal, the flies come swarming up out of the commode. I tried a half gallon of ammonia down the toilet and it seemed to help for a few days, but to no avail, they are back. I have talked with some of other motorhome residents in my park, but they have never heard of this problem. I usually dump both tanks when they are about two-thirds full. Has anyone had this problem before and what was the solution?
Jim, (San Antonio, TX)

Jim DID use his holding tanks properly! RV holding tanks are the perfect home for these flies, since they thrive on moist organic waste, especially solid waste. Just the kind of stuff you’d find in the black holding tank. Primarily one that has not been cleaned, flushed or maintained adequately. Chances are, at some point, his new-to-him motorhome was probably stored without the holding tanks being flushed, cleaned and emptied completely, and the previous owner left the black tank open when hooked up, allowing the invasion to begin in the first place!

So What Exactly is the Proper Use of the Holding Tanks?

When at an RV Park with Full Hookups, NEVER LEAVE EITHER TANK OPEN! Most campers do leave the black tank closed until it’s 2/3 or more full, and that is the proper usage, but they also think it’s okay to leave the gray tank open all the time, and that it absolutely incorrect!


This is the proper procedure for using your tanks at a full hookup site:

  • Check your fill gauge every day. We’re looking for at least 2/3 full.
    (You will usually have to dump your gray tank twice as much or more than the black)
  • When both tanks are over 2/3, dump the BLACK tank first. Let it run until it’s almost empty and close the valve. If you have a built in tank wash system, use it now with the tank valve closed and then dump it again.
  • Once the black tank is empty, open the gray tank valve. This will flush your flexible sewer line with gray water.
  • Disconnect the sewer line from the RV and flush it with clean water from your fresh water supply line. I use a garden hose splitter at the RV and a separate old garden hose to flush the sewer line.
    This splitter is very handy on site to have a second green garden hose available for washing tables, chairs and other things before packing up.
  • Reconnect the sewer line to the RV and keep both valves closed until the next dump.
What is the danger of keeping the gray tank open?

Simple! The flies com into your RV via the Super Highway flexible drain pipe you just connected to the underground system, where they live and breed! They come right up the pipe and into your tank, where the breeding ground is optimal for them! After all, all of your kitchen food particles go there, as well as your dead skin! A virtual buffet for drain flies!

Flies in the light
Flies in the light

Remember how flies propagate! They lay eggs that turn into MAGGOTS! These are prolific little creatures; they lay their eggs in masses of anywhere from 10 to 200 groups,according to some research. The larvae resemble small worms; basically without legs. The eggs can hatch anywhere between 32 to 48 hours. They mature in about two weeks and simply keep reproducing until they die or are eradicated. But keep in mind, new adults keep emerging from the pupae every 20 to 40 hours! The adults live approximately two weeks.

These little critters can infest and entire RV! They lay in the tanks and the larvae crawl… sometimes up through the drain and into your sink or shower! Just Google maggots in my RV bathroom to see all the nightmare stories!

Once in the bathroom, they will infest your walls, light fixtures, and just about every nook and cranny in the RV!


How to rid yourself of these pesky little creatures

There are lots of products that claim to work, however, I prefer to use a green product if available. The following products are used successfully to clean and maintain your tanks. The first product is specifically for the drain flies, and the second one is for use after every camping trip and before you store your camper for any length of time.

  1. Green Drain Fly Killer (link) Also excellent at eating residual scum – 4.5 stars
  2. RV Flexible Tank Pressure Washer (link) An absolute MUST for every RV

How To:

This procedure must be done while hooked up to a sewer.

  1. Pour ONE cup of Bye-Bye Drain Flies into your empty black tank.
    (If you have flies in the gray tank, pour 1/2 gallon into the empty tank and fill to 100% with water)
  2. Attach the Swivel Stick pressure washer to your garden hose and turn it on, leave the stick valve OFF.
  3. Open your toilet valve to get access to the black tank. Insert this rubber hose connector into the hole to keep the valve open. Run your Swivel Stick down into the tank and open the valve to pressure wash.
  4. Move the Swivel up and down all the way, from the bottom of the tank to the neck. Continue this up and down motion until your tank fills all the way to the bowl!
    TIP: If you have a bad fly problem, before you open the tank valve, have a shop vac on with the suction hose in the toilet to suck up any flies that come up.
  5. Once the tank is full to the bowl, go outside and dump.
  6. Repeat as necessary. Usually two to three times will solve the problem.
  7. Pour one more cup into your clean tank with once cup of Dawn dishwashing liquid. Add one gallon of fresh water and close the toilet valve.
  8. Move to your sinks, kitchen, bath. Pour 1/2 cup down each drain and run the water for five seconds. Pour another 1/2 cup and do not run the water. Close or cap the drains for 24 hours, or until your next trip or season.

If you have flies in the gray tank, pour 1/2 gallon into the empty tank and fill to 100% with water. 100% is when the water backs up into the shower. Let it sit overnight and then drain and repeat. ASSUME if you have them in your gray tank, they are ALSO in the black tank, since the tanks are almost always connected, so follow the black tank instructions while you wait.

You’re now ready for storage!

Questions or comments? Leave them below and I will answer them!