cleaning tank hoses

Are you talking about grey water hoses? If so, fill your sink with hot water, put in two table spoons of bicarb soda, stir and release. Do that once a week and bobs your uncle. The hose clogs up because of soap residue that clings and holds onto all the "gunk" you empty down the sink. Don't allow oil, food and grease down there and you solve most of the problem.
regards

So I gather your fresh water hoses are clear plastic. The reeson for the "Black Gunk" is algal growth which only occurs with exposure to light. The way to stop it is to first get rid of it by adding a bottle or perhaps two of white king chlorine bleach to the water in your tank. Or liquid pool chlorine will do the same job. Need a bit less though, probably only a cup full in your tank. Fill it with water and pump it through the lines then let it rest for an hour then pump it again and repeat until the tank's empty. Once you've done that and the pipes are pristine then you need to cover your clear plastic hose with that black plastic split tube they use for electrical wires.

Or you could just replace all the clear plastic pipes with drinking water grade black poly pipe.

yes i agree with the clean-out procedure mentioned but to eradicate the problem but stop it returning i would replace the clear plastic hose with proper blue grade water hose available from bunnings or camping /caravan shops as it's the sunlight that is allowing the algae to grow buying black tubing to cover the exsisting tubing is one way but you still have to remove the hose ends and clamps to install the black tubing do yourself a favor you will find in the long run the water will taste better coming through the blue water hose and you wont need black tubing over it and the algae problem shouldn't return
clear plastic pipe is el-cheapo water pipe and shouldn't have been installed in the first place unless it is a very old van you also need to leave your tanks full while in storage as algae needs air and sunlight to allow it to grow maybe your van was built prior to blue pipe been available upgrade is the way to go and you will be happy you did the balls in your court cheers

Milton baby bottle cleaner but as others said, you have the wrong tubing to begin with. Sunlight will ensure its return.

About 40 years ago, I worked on a station that had a 30K gallon water tank on a knob about 60-80 feet high, behind the buildings. As the tank didn't have a lid on it, the water often developed a fair bit of algae in it.

The previous manager advised me to use a quantity of Bluestone(copper sulphate) mixed with water, and scatter this over the water in the tank. Day or two later the algae had gone, along with that taste.

Can't remember the exact mix, probably 2-3 cupfulls in a bucket of water. Perhaps if you were to use a tablespoon of bluestone per tank, leave for day or two, then rinse thoroughly, your problem should be solved.

Bob