This may make an argument for pushing air up the tube with a fan. By doing so, you create higher pressure in the tube and the exiting air flow would discourage warm air from drifting across the field of view above the scope. Of course, it will be pulling in air from the mirror box area and this would tend to be much less contaminated by body heat.
Viewing in NH in cold weather I always had a problem with body heat screwing up my images in my 10" dob.
This is true, and is also the reason why Ed Grissom still uses sono tubes but with a truss, the air can not flow smoothly upwards the way a sono tube carries air up. This is the drawback to the truss design. I have always implied that a sono tube behaves like an air tunnel or shaft to keep everything constant with a fan at all times.