Adam,
To the average Beaver owner, intake manifold temperature is not a significant factor. The intake manifold temperature is significant when determining the efficiency of the CAC (charge air cooler) in relationship to turbo boost pressure. So if the engine temperature is OK, the engine power is OK, the air filter is OK, the intake manifold temperature will be OK. The only way that you can control intake manifold temperature is by reducing the turbo boost with less foot on the throttle. The "normal" temperature will vary between coaches depending on CAC efficiency and how hard the coach is driven.
Gerald