Drain the tank and then refill. If you park the trailer for more than a week or two, I'd suggest draining the tanks when you park it and refilling the tanks just before you head out.
Sanitizing the tank would be to drain, fill with a bit extra bleach, pump water through all valves until you smell the bleach, let sit for an hour or two, drain fill with fresh and pump through all the valves to clear out the excess bleach.
Keep in mind that plastic pipe and tank material really doesn't like chlorine, so keep it properly diluted. Your owner's manual should have a process with amounts and such.
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