These apps can work two ways....using sound, and using the internal
The frequency give you a hint of how big the thing is which is vibrating.
A lower frequency would be something like a tire or wheel, a higher frequency would be the bearing in a heater fan.
For a chassis vibration, watch the phone and try to make the vibration come and go. Watch which frequency seems to match when it appears.
then move the phone around the coach, to see when the amplitude is greatest (loudest). That means your getting closer.
Using your ear may work better.
2014 40QBH Phaeton DP Cummins 380HP ISL, Freightliner XC, 2014 Jeep Trailhawk TOAD
Blue Ox Aventa tow bar, SMI Air Force One -South Orange County Ca.