Smells from the waste tank

Hi all, we are just using our new van for the first prolonged period, a month so far, and we have just started to get a drainy smell up through the shower and sink wastes which is obviously coming from the grey water tank. I have probably allowed it to dry out by draining it too low but can anyone tell me the best way to get rid of the odours? We are in Spain at the moment and will not likely be able to buy tank fresh on our journey. We are careful about what waste goes into the tank so did not really expect this so soon. Regards, Roy