Don't know why people haven't leapt in to this one, as it's a well asked question.
Your handbook should tell you this, but this is my way of doing it. Drains is simple - bleach regularly keeps pipes and wastehog sweet. Watch out for pipes that have a droop under van, this can trap foul water.
Fresh water needs a pre-season cleanse. Make sure all of water system is drained, and if you have a charcoal filter, remove it and fit a blank (empty) one and then close valves. If you leave charcoal filter in it acquires a very bleachy taste and needs changed. First add 3-4 pints of white wine vinegar (buy it wherever you see it cheap) to bucket of water and fill up hot tank. Turn on heater and wait 1/2 hour. Turn off heater and run water through taps, topping up bucket as required. This gets rid of any scale in water heater just like you can get n your kettle, and should come out "milky". Really only needed in hard water areas, but we do travel around a bit.
Fill your water barrel/jerry with solution of water and sterilising liquid or tablets. I don't like Milton as it can leave a taste, but use the cheap supermarket tablets instead. Mix according to instructions. Give it a good shake to clean barrel. Run this solution into the system, including hot tank. Leave for 1 hour. Run taps, and be disgusted at black bits in sink! Remove any tap fittings like little filters on ends, to get out all the bits. Refill water barrel, and flush through again, and again. When finished open all the drains. I keep the first flush, ie with strongest solution, and fill and rinse toilet flush tank with it. This gets rid of black bits in there too, but be careful as Milton reacts with urine!!
By now you'll be fed up, so make a nice cuppa. Water from taps should smell sweet, with no chemical smell or taste. The water tank will be clean, and why not soak the filler tube and pickup for a while too.
I don't drink water from taps, but fill a bottle freshly. If on site for a week or more, I drop in one tablet of steriliser to a full tank of water once a week. This doesn't taste, but keeps bugs at bay. Obviously if you have a filter, don't do this.
I try to do this at end of season too, but usually run out of time. It's a good reminder to me to open all the drains, and remove bungs in tanks, etc, for the winter.
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