How to Clean and Sanitise Your Motorhome Water Tank

Your motorhome's fresh water tank supplies the water you drink, cook with, brush your teeth in, and shower under. If it's not clean, you'll taste it, smell it, and potentially get ill from it. The grey water tank, which collects waste water from sinks and the shower — brings its own problems if neglected, mainly a foul smell that permeates the vehicle.

Both tanks benefit from regular cleaning. Here's how to do it properly.

Why Water Tanks Need Cleaning

Standing water is a breeding ground for bacteria, algae, and biofilm. Even if you're filling from a clean mains supply, the tank itself can harbour contamination  especially after winter storage, when stagnant water has been sitting for months.

Biofilm is the main culprit. It's a thin, slimy layer of bacteria that forms on the inside surfaces of the tank and pipework. You can't always see it, but it affects taste, smell, and water safety. In the worst cases, poorly maintained water systems can become a source of Legionella bacteria, which causes Legionnaires' disease  a serious form of pneumonia.

Grey water tanks have different problems. They collect soap residue, food particles, grease, and skin cells, all of which decompose and produce hydrogen sulphide  the rotten-egg smell that plagues many motorhome owners.

How to Clean Your Fresh Water Tank

What You'll Need

• A purpose-made water tank cleaner or sanitiser
• Clean water supply (hose or containers)
• Rubber gloves

Avoid using household bleach. While diluted bleach can sanitise, it's difficult to dose correctly, leaves a strong taste and smell, and can damage rubber seals and plastic fittings in your water system over time. A dedicated motorhome water tank cleaner.

Step-by-Step Process

1. Drain the tank completely. Open the drain valve and let all the old water out. If possible, run the taps and shower briefly to flush the pipework too.
2. Mix the cleaning solution. Follow the dosage instructions on your chosen cleaner. Most require you to dissolve a measured amount in a small quantity of water first, then add it to the tank.
3. Fill the tank. Fill it to full capacity with clean water. This ensures the cleaning solution reaches every internal surface, including the top of the tank where biofilm often forms.
4. Run the solution through the system. Open each tap (hot and cold) and the shower until you can smell or see the cleaning solution coming through. This sanitises the entire pipework, not just the tank.
5. Leave it to work. Most cleaners need a contact time of at least 12 hours. Overnight is ideal. Some products work in as little as one hour check the label.
6. Drain and rinse. Empty the tank completely, then refill with clean water and drain again. Repeat the rinse at least twice, running the taps each time, until there's no residual taste or smell.
7. Refill for use. Fill with fresh water and you're ready to go.

When to Clean the Fresh Water Tank

Start of season: Always do a full clean before your first trip of the year. After months of storage, the tank will have some level of contamination regardless of how well you drained it.

Mid-season: A quick sanitise every four to six weeks during regular use is good practice, especially in warm weather when bacterial growth accelerates.

After long periods without use: If the motorhome has sat unused for more than two weeks, drain and refill the tank before drinking the water. If it's been longer than a month, do a full clean.

If the water tastes or smells odd: This is your tank telling you it needs attention. Don't ignore it.

Preventing Grey Water Smells

The easiest way to manage grey water odour is to not let waste water sit. Empty the grey tank daily if possible, and always drain it before any period of storage. Adding a small dose of grey water treatment each time you use the sinks helps keep odour under control between emptying.

Fitting a waterless trap or one-way valve to the sink drain can also prevent smells from the grey tank rising back into the living space.

Tips for Keeping Your Water System in Good Condition

Use a dedicated drinking water hose. Don't fill your fresh water tank with a garden hose or one that's been lying in the dirt. A food-grade hose stored cleanly between uses prevents contamination at the source.

Don't let water sit. If you're not using the motorhome for more than a few days, drain the fresh water tank. Standing water deteriorates quickly, especially in summer.

Insulate in winter. If you're using your motorhome in cold weather, frozen water can crack pipes and damage the tank. Drain the system completely if temperatures will drop below freezing, or insulate and heat the water system.

Check for leaks regularly. A slow leak under the motorhome can go unnoticed for weeks. Check hose connections, the drain valve, and any visible pipework at the start of each season.

For more detailed motorhome maintenance advice, Out and About Live publishes comprehensive technical guides.

Clean Water, Better Trips

Water tank maintenance is one of those jobs that's easy to forget until something goes wrong. A bad taste, a foul smell, or worse, an upset stomach on holiday. Ten minutes of cleaning at the start of each season and a quick sanitise every few weeks is all it takes to avoid these problems entirely.