If you are concerned, flush the tanks out with a Clorox solution BEFORE your next trip. Don't let the Clorox solution sit in your tanks over an extended period of time. The Clorox may attack seals and gaskets if left to sit for too long.
I wouldn't and don't. Better idea is to completely drain them and leave them empty. I do put a small amount of RV anti-freeze in each - this is suppose to keep the valve seals from drying out.