There are NO standards used for sizing axles or for designing TT frames. There's lots of tech. data for axle assemblies themselves available on the internet from Dexter and Alko but zip on frames.
Yes, some have found that the axles are under-sized and have been able to get the frame manufacturer to replace them. When looking at axle rating, you need to look at the unit's GVW and subtract the tongue weight. What may appear to be overloaded is actually okay. Sometimes it's the tires that are under-rated. Axles are typically sized to be able to "just" carry the actual weight on them.
Don't expect a dealer, a factory rep. or even the plant to be able to tell you much on tech. info. on a frame and components. A brochure will tell you nothing about frames and axles.