Professional means it was paid for, not that it was done by someone who knew what they were doing. Check the back wall of the bus after the next trip or if you can remember from the last trip, was there more dirt on the back than the sides? If yes, this indicates there is a vortex generated behind the bus and hot air from under the bus is being sucked up behind the bus, that hot air would go back into the engine compartment, not hot air escaping through that vent.
you could fit a narrow door the same shape as that vent that you open when you pull up for the night to vent the hot air out, but you would need a big air scoops and fan assist on the side vents to get the air to come out through that vent while travelling.
A double ended barrel fan like this
http://www.ebay.com.au/itm/Car-Auto-Veh ... 27e8727205 but a trip to the wreckers will find a better unit, or a bilge blowers like this
http://www.ebay.com.au/itm/In-Line-Blow ... 2c8c4cb10e venting out the side just where it starts to curve around the back with the outlet facing backwards would move a lot of the hot air out of that area above the engine, one each side would do the job fine.
T1 Terry