Your tanks should be vented to the outside via the roof, so smells should not enter into your RV. If they do enter from the tanks, you have either a trap or an air admittance valve that's not working properly in the RV. If there are odors escaping from around the sewer connections (probably between the hose and the campground sewer), then a trap in the hose, makeshift or otherwise, isn't going to help anything.
I try to get my sewer hose as consistently level as possible so that stuff doesn't collect in it. I'm also not usually hooked up to a sewer for longer than it takes to dump the tanks, as I (thus far) have not stayed at a place with full hookups for an extended period of time.