They make engine degreasers that come in spray cans. You spray your engine then use a sprayer to rinse everything off. I haven't used any in years, especially with the newer engines that just don't seem to get 'that' grimy any more. Still, I will use a garden hose with a sprayer and spray down the metal area round the hood, the radiator, and any area I can reach with a hose and try to avoid running water over wires.
In older engines, if you got the spark wires wet, often times one or two of the spark plugs would foul out. In today's engines ... you can't even find the spark plugs!
I would not use a power washer on an automobile engine (car, truck, or Motorhome), but would not hesitate to spray with a garden hose, and avoid hitting wires.