Both tanks can have odor problems. Odors from the black water are somewhat expected but most folks don't think about the grey water tank. Here there may be bacterial growth from a buildup of food particles and grease, particularly after storing the RV for an extended period. There are many products on the market that can help with this problem and, once again, your local dealer can head you in the right direction. The black water tank is another story.
Odors here can be noxious as well as obnoxious.
The best bet, besides following the procedures outlined above, is to use additives to fight odor in the tank. Once again, there are many products on the market. This is where you need to talk to not only to your local dealer, but other Rvers as well to see what they recommend. Many RVers are adamant about avoiding products that contain formaldehyde. Although you can purchase toilet paper made specifically for RV toilets, there are alternatives.