[netjam]

Re: Odour....check under the sinks for the odour. I've had a bunch of vacuum breakers go bad and they are located under the sinks and allow tank odours. They are high up on the 1.5" black drain pipes and screw in, hand tight. If you don't know, they are cheap ($2) black plastic plug type fittings that dead end a pipe. If like you say the odour only occurs when your dump valves are open, my advise won't help.