I've found that the best way to deal with the water tank when not in
use is to drain it after your camping trip and leave the top cover,
drain cap and filler cap open for a few days to let it dry out. I also
use a clean towel to soak up water on the bottom of the tank. Run the
pump until it goes dry. You can also use a hair drier with heat off.
If the van is parked outside, you may want to leave the filler cap
closed to keep out crud (bugs, dust, etc.). Makes sure to put the
drain cap back on if you plan on driving the van. This method
guarantees that no mold or algae will grow in there and has worked
well for me in the last 15 years or so.

Bleach treatment. Not everyone will want to use bleach due to it's
toxic and caustic nature, but if used with caution it works very well.
Now, if the tank needs cleaning, I will carefully pour some bleach
into the tank using the outside filler. It is not a good idea to bring
bleach into the van because it can easily ruin fabrics, etc. I use
about 1/4 cup mixed with a few gallons of water. Carefully pump some
of the mixture into the sink using an old towel to keep any splashes
contained. Let the mixture sit in the tank for 10 minutes or more.
Drive around to get it sloshing. Drain and rinse. Let it dry as noted
above. Your tank is now clean and bacteria-free.

By the way, 1/2 teaspoon of bleach can be added to a full water tank
to keep it from getting nasty. If you are treating water from a lake,
stream or shallow well, use twice as much household (5%) bleach and
wait twice as long before drinking it because it is more likely to
contain chlorine-resistant parasites from animal droppings. Let the
water stand for at least an hour after adding the bleach before you
start drinking it. If the water is colder than 10�C or has a pH higher
than 8, let the water stand for at least two hours before drinking.

YMMV

Cheers

Keith O