Try
Yahoo Groups. I saw stuff for a '04 TropiCal there, and I posted links to '05 Dolphin manuals. National RV used common systems and components throughout their models, so you might be able to find an answer to a question by looking at other models that are close in year, if you can't find the one specific to your model.
Freightliner is still in business. Did you check their web site for information?