Bill,
Actually both statements are correct.
There were some gas coaches where the limit was pushed by the addition of a tag axle. Both Workhorse and Ford addressed this in the late 1990's by increasing GVWR's to accommodate heavier coaches.
But, those early gas coaches are really unrelated to the larger diesel coaches that were designed from the ground up with a tag axle.
Would I own one of the early tag axle gas coaches-- NO. Would I rule out a diesel because of a tag-- NO. On a diesel it is often a tag or inadequate CCC.