We have a R-25 Classic. The cabin is not airtight with respect to the cockpit. There are small openings along the inside of the gunwales between the cabin rear wall and cockpit. The openings are large enough to run wire. Since the engine extends forward of the rear cabin wall into the cabin there is air leakage around the rear cabin step.
All that said, we do not notice the smell of diesel when underway unless the rear cabin door is open. The carbon monoxide detector in the cave ever registered anything other than trace (under 5 PPM) amounts of CO. But every couple of years I notice the mouse fur near the opening in the cave has slight diesel staining from fumes.
While not the R29, of all the RT models built, ours is the one that is arguably the most susceptible to smelling diesel in the cabin while underway.