As you can see, the aircraft is in a very nose-high attitude and it is obvious that he is in a high drag situation, most likely with full power. One might suggest that it should be obvious to the pilot that the speed is slow, and I'm sure it was obvious from an airspeed point of view but not necessarily from the view out the front window.
The view ahead on the Pawnee can be quite misleading. I have Pawnee experience towing gliders. If you take a look at a side view of this aircraft type, you will notice a significant slope downward for the top of the cowling.
https://www.google.ca/search?q=piper...&bih=929&dpr=1
The result is that level flight has a very nose-low appearance. I have flown a variety of tailwheel aircraft and most of them have a very nose high feel to them when doing a three-point landing(as much as completely blocking the forward view in some of them). The Pawnee is very different with a nose-low view when in the three point attitude. Therefore, it seems to me that it is more important in the Pawnee that in your typical GA aircraft to monitor your airspeed when in slower flight regimes, especially if you are not used to this aircraft as one can accidentally get slow when the view out the window does not make one feel that they are at a high angle of attack. If that had been a Cessna 182, I suspect his forward view would have been blocked while towing that banner the way he was.