The method above will work to pull warm air from the dash back through the motorhome.
However, you're using outside air from the front all the time to maintain this flow. Thus you are also pulling into the motorhome outside dust continously as you travel. And of course you cannot have the dash controls set to "Inside Air" (to keep out dust) and have this flow system work.
We keep all motorhome vents and windows closed when traveling - and use "Inside Air" for the dash controls as much as possible for both air conditioning and heating to keep cleaning maintenance due to dust and dirt at a minimum.