Hi Rex,
Is it carpeted? I recently cleaned the carpet ceiling in my '96 Adventure. All I did was take everything out of the cabin, including the compartment lids, and then spray it down with a pressure washer. The water flowed down the compartments, into the hull, and out the stern plug.
Of course, you'll want to start with the top, and you'll want to angle the 'gun' in the direction that you're moving it, as if the water were 'scraping' along the surface of the carpet. Then as you approach the wall, angle the gun downward, as if you were forcing/scraping all of the dirt down the wall. It was a wet, nasty job, but in the end the carpet looked as good as new. It also dried in just one sunny afternoon with a fan blowing in the door, and with the windows and hatch left open.