We are fortunate in that when we aren't using our pod we can park it in our horse barn, out of the sun. My concern is you are under trees is the sap that drips on it day after day. We have had some pretty nasty sap run on ours during camping so if are given a choice we camp with no trees overhead.
We use well water, once it is in our tank come April, we just keep on filling it up during summer and don't drain out until we are done in mid-October. Last time we used bleach in our tank was April 2012. I am no expert on this but it would seem if you use city water there are already a host of
chemicals added (some ok, others questionable) so it should be ok to leave in the tank.
We keep our slide in when not in use. Better seal for keeping bees, wasps, hornets, snakes out (they like the slide area underneath) and don't forget to cover your tires so they don't get "sunburned"

really, the tires will last longer if not exposed to sun day after day.