do you keep water in holding tank between trips?

Generally nothing. We rarely use our FW tank and if we do, it's just enough to use the facilities while in transit. We did use it quite a lot on a longer trip and forgot to drain it. Kept trying to figure out where that nasty smell was coming from. Bingo! Time to sanitize and we're much more careful to remember to drain it when we get home. We never seem to have an issue with whatever water might be left in the lines or WH.