Camco EZ Level RV Bubble Level

Sometimes the simplest tool is the best one. These stick-on bubble levels let you check your RV's level from outside in about two seconds. No app, no batteries, no pairing. They are always on and always accurate. At under a dollar per level, there is no reason not to have them. The adhesive holds up well in weather but consider adding a dab of silicone for permanent mounting.