We clean it when it gets dirty or too wet. The fan we have in the chest freezer helps, but 25CF is a lot of volume for a little computer fan to dry out.
When you start kegging, you will drip beer in the thing. It's just a fact of kegging/chest freezer life. Just mop it up before it gets nasty.
I like Swiffers on a stick to clean the bottom. I should clean it this week since it's gotta a little wet recently, but we will see how that pans out. Still have to stain the deck and that takes a bit of precedent.