I would have to think that if there is material being projected from the p-traps, it is a poor design of the plumbing. To project, you would need to build up sudden pressure from the tank material sloshing towards the area the pipe is connected to the tank. This is why the plumbing should only tap the tank top near the center, never on the ends or sides.
Back to the OP question. We use to have the issue in our old 69 Franklin on occasion. We usually would find something in the rear of the camper not properly closed creating a vacuum on the camper pulling the smell into the unit. Until we could stop to investigate, we would crack a window towards the front of the camper to equalize the pressure.
As a kid, it was ride in the TC or have 5 people in a regular cab that wasn't safe by todays standards. Today, with all of the safety designed into the cabs, there is no way I would let my kids ride in the back.