If you read through Load Rite's documentation on hot dip galvanizing you can see that it does galvanize the inside of the box section as well and from everywhere I can see the inside of the frame on my old '94 is galvanized, or else I would probably not have had it this long!
But I have seen galvanized trailer frame parts rotted out, mostly cross members probably because they did not have a drain hole drilled at the lowest point, and sometimes on axle assemblies because they were not welded fully so water got inside. I had a 2x2 galvanized axle that I just replaced, it was built by Champion Trailers and installed in 2004, it had hardly any rust anywhere and I was able to sell it for only a little less than I paid 17 years ago! So galvanizing like many things depends on workmanship. I replaced that one with another built by Champion Trailers, 6000 capacity, drop/saddle design.