Neither our van or the places we stay are level. But how level do you need to be? Years of camping had taught us to always have our head high when picking a campsite. Years of living in a house said we need to be level while sleeping.
When we first started in our van, we would move the pillows to whichever end of the bed was higher. Remaking the bed to change ends was a pain especially when we were tired. And sometimes our camping spots just were not level enough.
To fix this problem, we purchased these drive-on levelers. When our van does not seem level, we drive up onto them.
Initially I used an app on my phone and placed it on the counter to see if the van was level. After a few nights where we still slid down or even out of bed, I changed to using a real, bubble level. I push up the bed, placing the level on the bed platform.
We generally find that as long as some of the bubble is in the level area we sleep well even if the van was tilted slightly the wrong way.