There's not a whole lot of ways to do it. Probably the best is to just add some water, a bit of bleach and go for a nice bumpy drive. Somewhere along the way, open the drain valve and let it all drain out. Repeat this a couple times in a row, then rinse with fresh water to get the chlorine residue out. Fill the tank completely full, and drain it to clean out the chlorine. While you're at it, you might as well run some of that chlorine water through your pipes and sanitize them. Other than this, I don't know of any other way.