To take care of the immediate problem you will want to locate a product called Happy Camper. This WILL kill the odors in your black tank like no other product I have ever found.
That's just a band-aide for the actual problem. If you are running an exhaust fan when flushing and you notice the smell coming in then your vent is clogged. If you don't have windows open and the exhaust fan is not running and you still have the smell coming up through the toilet dump hole (yes, you need to sniff around) then you have a serious problem which, again, might be related to the vent.
A normal black tank without any chemicals at all should not allow noticeable smells back into the RV. Not knowing what has been in the tank in the past also leaves areas where some additional steps might be required. If you can find a service called All Pro Water Flow
http://www.allprowaterflow.com/ have them return your tanks to factory clean and go from there.