The HWH Specialty Hydraulic Oil is clear while the Dexron ATF is red. If you were to develop a leak in the coach from one of the slides the red fluid is liable to stain whereas the clear fluid won't.
Our 01 Adventurer came with Dexron II in the leveling/slide system reservoir. There's a sticker on the top of the reservoir that says "Use Only Dexronc II Automatic Transmission Fluid". The HWH manual says to use only the clear HWH oil.
Last summer at the GNR I asked a Winnebago rep which was correct. He said that Winnebago had been using Dexron for several years and felt it did an excellent job. The only possible downside was if a leak developed and fluid got into the coach it could stain the carpet or wall covering.
In our case there aren't any hydraulic lines running where they could leak into the interior so I stick with the factory recommended oil.