How do you know if your van is level?

hitchglitch

We have a (front to back/side to side) level gauge near the door and a small hand held gauge but every flat surface in the van gives a different reading so we think that we are level and after a few hours we notice that the bathroom door gradually swings
open or closed!  I guess we need to 'calibrate' the van and find one position for the gauge that seems right.  Suggestions?