Eric, just a couple of thoughts.
I would not be too concerned about shade blocking the panel after 3pm.
IMO, you would be beyond the "peak" period for solar. As you know, the lower the sun is, the less wattage you'll get. After 3 pm I would think the impact of shade is minimal with even less impact if you run two smaller panels. One will be shaded and the other will have sun.
If you have the room on each side of the cab-over escape vent, you might benefit from installing two 80 watt panels . One on each side.
Here is a thread offering some good info once you decide on the panels. Check amazon for prices.
solar panel thread for Lance .