take a vacuum cleaner to the roof and use it to blow very briefly in the vent pipe..... if you get a smell in the trailer, then you may be able to sniff around until you locate the source..... and possible blow out any cobwebs that may be in the vent, as cobwebs will limit the flow of passive air movement, causing the vent pipe to become ineffective.......
if its determined that the smell you are referring to, is not coming up out of the bowl when you flush, the only other place it could be coming from is the flange area, but the smell of a leaking flange usually lingers around for awhile, UNLESS its leaking in such a way that only air is escaping...
when the contents in the toilet bowl gets flushed down, it has to displace the air in the tank, and if the vent is not working or is plumbed wrong, it will purge the tank air up and out the toilet when you flush...... a good test for this is to fill the bowl completely with water.. then flush. if you get an air bubble up from the tank, you know the vent line is NOT working like it is supposed to.... if the mass of water flows freely down into the tank, the vent is working well...
