The fan was turned on at 4pm and off at 8pm. That's an extra 4 hours, totaling 17 hours off just two D cells. That's really quite impressive! At the same time, I set the new fan going, I set the undersized CPU fan going. Next I went outside and checked airflow. Both seemed fairly similar though inside the noise from the cheap fan was horrible.
I have very much the feeling that there's nothing really to choose between a silent CPU fan running off 12v and about 160ma and a 3v Walmart fan using higher ma. I suspect watwise they'll be about the same.
At 8:30pm I checked temperatures and found 93F in the galley, 94.8F in the bedroom and 97F in the cockpit. I'm pretty sure the fans are sucking hot air to the back but cold air is not replacing the hot. I suspect I'm going to have to install a fan to pump cool air from under the bus. Without a black tank, that should be possible. If I install a black tank, it might become too smelly to draw air from under the bus.
I did consider side vents but in rain, water might be drawn into the bus, even if they were correctly louvered. A few days ago, in Big Lots I saw some filters that looked the right size. Sadly they were heavily laced with chlorine which would not be ideal.
Meanwhile, I ordered a pair of 10w weather sealed solar panels. My thought is they could be used to power 2 CPU fans. My quick calculation is that two 160ma CPU fans would use 2w of power. I already know they'll run happily off my 5w panel though the starting current is higher. One panel might do it but I doubt it. Putting two plus some lithium batteries should provide for 24 hour ventilation. That's the theory. I'll have to see how it works in reality.