Given your Webster's dictionary definition, I would say it is uncommon.
It is not "usual" for a water tank to fall out.
But does it occur frequently or habitually? I think you need definitions for those words as well. What is frequent? Given the hundreds of thousands of RVs on the road, I would guess it happens more often than you may think. But as a percentage of total RV owners - probably pretty small percentage.
As Dirty Harry said: "Do you feel lucky? Well...do you?"
If yes, then don't even peek down there.
If not, then an ounce of prevention is worth a pound of cure.