I'd bet the previous owner didn't properly care for the holding tank "Biome" and instead tried to cover it up with a more powerful scent... Unfortunately there is lots of advertising for products like that. look around the compartment for a scent device, the thing could be attached or simply rolled around to some less accessible area. I got my used camper and it smelled like formaldehyde (from the cheap/unfriendly holding tank additive).
With that said "When in doubt - Air it out!"