I actually do not agree with the portion of this that indicates that there is a designed in cooling problem.  I also do not understand just what you mean by "... not enough air flow in the slipstream ...".  What do consider the "slipstream"? Sides, bottom, rear?

The coach was designed with a rear engine and a rear radiator that does not receive ram air.  The engineers adjusted the design to allow for that fact.  The primary adjustments were to provide a large radiator and a large fan.  The fan pushes air from the engine compartment through the radiator while the low pressure area at the back of the coach helps draw air through the radiator.  The primary source of incoming air for the engine compartment is from under the coach followed by the side vents.

Provided that the radiator is clean and the fins are still attached to the core and viable AND the spaces around the edges of the radiator are blocked so that air can only exit through the grate and not recirculate around the edges of the radiator, no other provisions need be made to have the engine properly cooled by the stock design.

Coaches that have a cooling problem usually have one or more of the MAINTENANCE problems noted above.  Side radiators are good.  They can receive slightly cooler air, but need to be powered in some manner and are more expensive than a rear mounted radiator.  In a large coach with solid rear axels it also allows the rear axel to be moved closer to the rear of the coach for better balance and handling.  Due to the independent rear suspension of the FMC, the axel in it's case, is already far to the rear as compared to other rear engined coaches.

Bottom line: most FMC's that are properly maintained do not have a cooling problem.  Most of the ones with a cooling problem have a maintenance problem.