You're right about CAT having to be "legal for trade" in terms of calibration. A mobile scale could have same requirements ---or not--- depending on how it was to be used.
Few of us search out "corner weights" and just use axle weights like CAT. Michelin recognizes that RV corners can be a lot different so their RV version of the inflation chart is by Corner. Single Front or Pair of Rear Duals. But they go on to tell us to use the higher pressure called for by the heavy corner for the lighter corner as well. So say the chart calls for 70PSI left and 60PSI right, you'd use 70PSI for both corners.
To part of your question, had the CAT Axle weight matched the sum of those two Corner weights, I might be a little more concerned. My thought is the CAT weight will be more accurate than the mobile.