Check Van Level is an app that works in conjunction with a sensor to help you quickly level your trailer/caravan when you are setting up.
To calibrate the app for the first time, the sensor should be mounted inside the van and plugged into a USB power outlet. The van should then be levelled using a spirit level. Once this is completed, the app can be opened, click on settings and then the calibrate button. This will remember the van's current level and use it for future reference.
Once calibrated, the app can be used to check the van's level, by aligning the arrows on the side and rear views. When the circles turn green you are within 1 degree of level.
You can optionally enable the height and angle feature to get a more accurate reading of how far off from perfectly level the trailer is.