I would not say it has to be EXACTLY level. You don't want it to be on too steep an incline. The first trip we started on we were in a steep driveway, started to pull the slide in and it went 'down the hill' so to speak and stopped! I pulled out of the driveway and it went right in!
I was always told, and have stuck to it, that you should be one half a bubble count (the rings on the circular level) to the curb side. This is for run off of the AC to the other side. This was my biggest complaint on my last setup as it was constantly emptying the AC water off on the door side and usually on to us. Its not really noticeable in the trailer but is enough that it works as I did it this time with our new 5er.