If you just bought the unit I assume the black tank is empty. Why not add a couple of gallons of water and a good quality black tank chemical. If the smell continues after that you may have a clogged vent pipe. If the PO hasn't been taking care of those tanks they also sell a grey water chemical that helps with that odor. Walmart carries several brands of chemicals. If you still get the odor after putting in chemical and checking vent stack it may be that PO let the black tank back up and never really got the tank empty. High pressure flushing may be needed. Odor like you mention is not normal.
