Water runoff control is a very good reason not to have a pad in a campground perfectly level, and as said probably one of the major reasons they get build slightly off level. Sometimes settling or frost can make things that were once level less level as time goes on, as well. For many loose surfaces like (some) gravel or dirt or crushed stone, just driving back and forth can make ruts and divots and so forth that cause some unevenness.
My class C isn't perfectly level inside when the wheels are perfectly level outside; the house tilts nose down compared to the ground a tiny bit. I think that's not at all unusual. For a perfectly level pad, I need one set of my 2x ramps under the front wheels to get the house level. (It's also somewhat unfortunately the case that not everything in the house is mutually level or plumb or square with the rest of the structure....)
I suspect a gimbaled bed would cost more than a couple hundred bucks extra, but then again I've never attempted to design or build one. Maybe a hammock arrangement would be the best way to go.