I don't have a motorhome, but our old Chevy crewcab pickup had a 454. If your engine cooling fan has a clutch on the hub, it could be worn out and slipping. This causes the fan to free-wheel and turn slower than it should, and it won't move enough air to cool the engine.
If you are familiar with under-the-hood sounds, is the fan quieter than usual? In the summer heat, at a fast idle, it should sound like a propeller plane taking off. If the fan is only moving about the same amount of air as at idle, it is a good possibility the fan clutch is bad.