It is what it is. Most of the time it is clear ATF Transmission fluid. Sometimes it is hydraulic oil. Lippert says for filing, any ATF can be used. A full synthetic or synthetic blend works best such as Dexron II, Dexron III or Mercon 5.
Source:
http://www.primetimerv.com/download/...viceManual.pdf (Page 12).
If you have the Level Up system they now reccomend replacing one qt of ATF (or adding if you are low) with anti-stiction fluid to eliminate/reduce the popping noises.
http://lci1.com/images/support/lipsheet/0295.pdf