We have a class c and in the summer always have windows and vents open when travelling.
Sometimes you do not have the option of dumping the tanks where you are staying.
I just drove 40 miles with a 1/2 full black tank (to get to the closest sani station) half that drive was over rough logging roads, no smell. We use lots of water, and tank deodorant (brand vary).
Something to check, our bathroom sink drains into our black tank, not grey. If the plug is left up, we can get smell. We always leave it down, just pop it up to drain the sink.
As others mentioned make sure there is water in all the drains.