Lots of varied and opposing views will come from this question. I used lots of different chemicals as I did the learning curve. What I found was...pour in whatever chemical I was using, in a week..oh my God...don't use any chemicals, make sure I have 5-7 gallons of water in the tank after flushing...in a week..oh My God.
Bottom line is...if you are moving the rig, after a week, it's going to smell. You will get the gases as the Rv goes down the road, windows open, etc...draws the gases/smell into the cab. And yes, if you are using the RV plumbing you will have water in the P traps anyway. Don't wait till the tanks are full..just do a dump and clean once a week. Takes care of all of that.
Gary Haupt