I have the same engine with a rear radiator in a 2003 Safari. Never have had a over heating issue in 41,000 miles. Here are some thoughts:
With the radiator warm, spray some Simple Green into the radiator and let it sit for 15+ minutes then spray it down. If you have a lot of gunk in the radiator this will help. Do not use high pressure water as you can damage the radiator veins. The other thought i have is that in most coaches with a Cat engine and an Allison transmission they are tied together with a common cooling system. Does your transmission also get hot when your engine over heats? If your transmission also gets hot you may have an issue with the transmission and not the engine.
