Keeping it level, that is our understanding,
While traveling, not a problem, as it is rocking and rolling, UNLESS you stop for lunch or siteseeing and park in a really unlevel place.
While set up, if you have the rig reasonably level, doors don't slam shut when opened, and you can sleep on the bed without rolling off, they say you are OK.
While in storage, some people park rigs at some terrific angles, we are a little amazed sometimes to see how people park them for extended times. Our preference is to keep rig level all the time, and when stored we try to level the same as if we were going to be living in it.
Years ago we would occassionally see a camper with their frig outside at a campground. They would be "burping" it. Set it on one side for a few hours, then on the other side for a few hours. The theory was that they could unblock the refrigerant and get it past the air bubble that they claimed had formed. Not sure it ever worked, but was a sight to see.
With units with slides, don't you want them to be sitting pretty level so that the slides work properly??
We keep it pretty level, that said, you don't have to spend 6 hours leveling it at every campsite. (Silly Carol, but you get the idea!)