Do not know how much help this will be. I was looking at a unit that had the side radiator, and the belts were super easy to access and the system looked easy to work with. With a rear radiator, I can only think this would be more cumbersome. However, if you have access to the belts ( underneath maybe or through the engine door ) the belts itself should not be that bad to replace. Either there will be a tensioner, or one of the pulleys ( or accessory like alternator ) will adjust and swing to allow for removal/replacement.
Do not know how easy it will be to find the correct belt. However, there are numbers on belts that dictate size etc, and you may be able to use that information when ordering a new belt if it is not able to be looked up by Freightliner/Spartan.
Agree with the above, could be under some type of chassis / powertrain warranty.