Does the new trap have a vent? If not, it may be getting siphoned dry. Slowly pour some water down the drain and see if the smell goes away. If it does, the trap is likely getting siphoned out.
Does the tub gurgle when its drained? May be a venting issue.
If it's exposed you will have your trap,then the vent and maybe next your waste pipe from the lav drain.Are your sure it is sewer gas?