lol, the mare I used to have was like that. She LOVED her bath and would come running to me at the site of the scrub brush and the garden hose.
When I used soap, I used mane and tail usually, but a couple times I just washed her in Dawn. She had a pretty brittle tail at one point, so I have her an oil treatment. I put a human hair mask on her tail and bundled her tail up in a plastic grocery bag for a couple hours before rinsing. She looked pretty silly, but her tail was very nice afterwards.
She liked the water so goo that she usually got a good hose down each week. I didnt soap her up every time, but would wet her down and take a very stiff house hold scrub brush to her. She loved it and would moan and groan and carry on.
Other than the time her tail got so brittle (due to some meds she was on) her coat was gorgeous. She stayed soft and shiny, and kept a loooonnng mane!