I went in with Butch on that roll of breather material and just put it in yesterday. Since we haven't left for the expo yet I can't say how well it works but I followed Butch's plans and lapped the material up under the headboard into that dead space so air from that end of the bed should be able to move under the mattress. My only concern would be that air is cooler and as it sinks under the bed it could cause an interface of cold dry with warm moist and could cause condensation under there where it mixes. The flip side is the cold dense air will help with air movement.
In any case, it's in and I'll see how well it works soon. We haven't seen condensation problems under the mattress in the Lance but after a bunch of years with the S&s I definitely saw some. I expect when we are full timing we'll see it more.