Dear Dave,
The cold water line in the front bathroom sink smells like rotten eggs, and it only takes 12 hours to smell that way. None of the other lines in the RV do this. We are full-timers and have been in the rig since it was new. —Phil, 2018 Heartland Cyclone 4151
Dear Phil,
According to the floorplan I found online, your Cyclone has the side slide bed in the front 5th wheel overhead with the bathroom at the front. The bathroom has the toilet and sink on the driver side and the shower on the curb side.

The puzzler here is the cold water line would come off the city fill or water pump and go through the water heater. A “T” would split the line to all the other sinks, shower, and toilet. So, why is it just the front bathroom cold water?
Some questions about cold water smell
You state that it takes only 12 hours for it to smell. Have you tried sanitizing the system and that is when it takes 12 hours, or just empty the tanks? Does it smell on both city water and fresh water tank?
Since the lines are most likely PEX material and with crimp connections, it is a closed system. There would not be anything coming from a compartment or holding tank. So it has to be that the water in that line is getting what I call “skunky”—which is typically caused by algae or bacteria.
What to do about cold water that has rotten egg smell
I would start by putting bleach in the fresh water tank and filling it up with water. The Centers For Disease Control (CDC) recommends 1/3 cup for every gallon of water, which I feel is a little strong. I have used 1/2 cup for every 30 gallons, which seems to work well.
The challenge is getting the bleach in the tank. If you have a gravity fill, you can pour the appropriate amount of bleach into a gallon jug to dilute it initially. Then use a funnel to get it into the tank, and then use a hose to fill the rest of the tank.

Some units have a valve at the city water connection that diverts the pressurized water either to the city water lines, or to the fresh water tank to fill it. When I was at Winnebago, we had a few years that units had no gravity fill, just the switch. So it was a challenge getting bleach or even antifreeze into the tank. A classic example of designers and engineers that don’t actually use the units!

In this case, you need to use a funnel to fill the end of the fresh water hose before attaching it, then connect it to the city fill. It should get the appropriate amount in the tank.
Next steps to get rid of smell
I suggest driving the unit around a little to mix up the water and bleach, then turn on the pump and open one faucet at a time until you get the smell of bleach. Do this to all faucets, shower head, toilet, toilet spray head, and exterior shower. Then let it sit overnight or at least 12 hours, and then drain the tank by running all the faucets.
Some suggest just putting a partial amount of water and bleach in the fresh water tank rather than filling it up. However, you could have algae and bacteria on the upper portion, and I believe in getting it all.
If the smell comes back after that, I would suggest replacing the water line going to that sink as it might be decomposing. You should be able to access it from the underside compartments.
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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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