It’s not like they use a lot of different sized axle tubes… going up in size will most likely have the same size axle tube… often with just a different bearing set or different spindles and bearings…
Almost all of the axle tubes that are used for our RV’s are 2 3/8 OD with a 3/16 wall tube, and rated at 3500 lbs.…
Above 3500 lbs. up to 5200 lbs. will almost without exception 3inch tube with a 3/16” wall…
When you get into 6000 and 7000 lb. axles you may run into be 3inch tube with ¼” wall…
Of course there are smaller tubes for lighter axles that we don’t see to often…