If you have an odor, weather from the gray or black tank, you don't need to add chemicals, you need to fix the problem that's causing the odor. Adding chemicals is just masking the odor and is just wasting money.
Before you travel, put a little liquid laundry detergent down the toilet and several gallons of water. The swishing will clean the inside of the tank. You can also do this to the gray tank. A little Calgon water softener added to each will coat the inside of the tank and make it more difficult for solids and other stuff to stick.
Good luck
Ron