There are two ways ti approach those stick-on bubble levels, either as an indicator or a reference. An indicator method is that when the bubbles are dead center, everything is correct for camping. A reference system means that the bubble can be slightly off center from frame or floor level, bringing the best results for whatever you deem most crucial for leveling.
FWIW, a Bluetooth receiver with indicator, either audible or visual, can be used with most phones while the driver is operating the vehicle and the phone is held on the trailer. I've been shopping for one with audible tone but it isn't one of my high priority items as my rig can be off level by a good amount and still be comfortable. I do have a set of stick-on levels and always carry a stick level.