Cleaning genset

I did not even know there was an electronics board under the mud that was on top of it, had no idea at all. It still worked just fine. Mine was five years old and not a problem.

The boards in an Onan are covered with conformal coating. So basically waterproof. What isn't waterproof are the terminals in the connectors. And a water short between connections wouldn't be good. So disconnect the battery first, spray the area with a bit of water and Dawn, rinse completely, let dry completely, pull the connectors and shake any water out, blow them dry with compressed air, maybe sit overnight or several hours in the heat of the day without connecting the battery. Then you should be good to go.

Try electronics cleaner spray from an auto shop.

I use dawn dish soap and water to clean off gen set. Spray WD40(Water Displacement formula 40) on connectionsbefore and after cleaning.

I spent my career repairing electronics (component level). I strongly suggest you do NOT use any type of circuit board cleaner or solvent. It will very likely attack the conformal coating and you don't want that. Don't spray it with high pressure air either.
Frankly, I would leave it dirty. It wont hurt it and getting it wet (esp. certain components) can easily lead to latent failures.

Cleaned my genset panel once on a former MH. A week later the board fried. Due to cleaning the seal was compromised and moisture got inside.

A broom/dry toothbrush/chip brush and maybe a damp rag would be about all I'd do. A little bit of dirt on the outside of wires or electronic modules won't cause problems, particularly in those designed to be used in a decidedly not dustless environment.

I douse mine with Simple Green, let it sit for a few minutes then hose it off. Wipe it down with a rag then turn it on and let it run for 10-20 minutes. I sometimes turn on the A/C so the genset gets some exercise. After that, I put some wax on it and it looks like new again. Never have had any issues with it. After all, it sits in a wet environment when you drive in the rain. Why would water from a hose bother it...YMMV...Dennis

I use the gunk Forum cleaner and then spray it with car wash a couple of times a year no problem so far

TO answer your question.. YES there are drain holes.

Though your generator may be different the only "Exposed" not waterproffed connerction on my Onan would be the 12 volt positive and water wont bother it.

But the compartment does have a hole. in fact about 4" in diamater. as well as some smaller ones.. It is how you cange oil filters on mine. Yours may be different.