The rear engines in diesel pushers get blasted with water, rocks, sand, and other road debris from the wheels every time you drive. The air intake and exhaust are piped to the outside of the vehicle so it is impossible to get water into those from the engine compartment. I wash my engine as needed and never had a problem over many, many (40+ ) years. As suggested by others, I start the engine immediately and drive/run the engine till the engine is dry.
If you have a rear radiator engine (I hope not as they are a problem), you will eventually need to clean the radiator from the top of the engine because the aforementioned road debris will clog the radiator and cause overheating on long pulls. When you try to clean this you need to cover everything in the room above the engine as the blowback will cover everything including the operator. For this reason, I let my mechanic do this task.
This brings up the caution that if you clean the engine from the bottom, take care not to directly spray the hatch seal. These are notoriously leaky and you might cover the room above in spray.
Have fun with your clean engine. By the way diesel engines seldom NEED cleaning. I clean mine to make it easier to work on and keep the turbine linkage clear.
My wife and I would travel full time, but we have to sleep occasionally.
2001 Tiffin Allegro Bus 40 ft DP named the Wild Eagle
Followed by the Eaglet - A 2014 Jeep Grand Cherokee.