As a long time Corn snake owner, I'd be willing to bet the retained shed, dehydration, and thin appearance are all related.

If a snake is underfed, it will not shed properly, and show signs of dehydration.

I have a Corn approx 48-50" in length, and I feed her a small f/t rat approx every 8-10 days. I noticed a BIG change in her bowel movements (more substantial) and her overall physical appearance when I moved up to rats.

I generally follow the "Girth Rule" which is the belief that feeding snakes prey items the equal size to their body girth is appropriate.
Its a general rule, and not always one to follow, but with Corn snakes, Ive found it can be applied with positive results.

As far as soaking, Id definitely provide a water bowl large enough for the snake to immerse itself in.
I would also soak it in a tub (bath, sink, or plastic tote) Usually when I soak my snakes for retained sheds or dehydration..I place the snake in an old pillowcase, tie off the end, fill the tub with lukewarm water about 1/2 way up the side of snakes body (so it doesnt have to swim constantly) and let it soak.. I usually wet the pillowcase down gently, and the snake will rub against the pillow assisting in removing the shed. If it has severely stuck shed, I will continue to soak the snake for as long as it takes. Just warm the water periodically to ensure the snake stays in lukewarm water (not too hot...if it feels hot to you, or warm, it could be TOO hot)

After about 10-20 minutes of soaking, I add a few drops of vegetable oil, mineral oil, vitamin C oil, or olive oil to the water. This helps loosen any sticking eyecaps or skin, and is safe for the snake.

I would recommend a nice soak at least once a week, but wait a couple of days after feeding so the snake does not regurge.

Hope that helps