I haven't documented an API but I do have a good knowledge of RoboHelp.
It's difficult to respond without some knowledge of the structure of your current document. Would I be right in thinking it probably consists largely of tables with items in one column and then some more columns that explain functionality? Could you post a screenshot or send me something offline via my site?
Would I also be right in thinking that certain items are repeated in various locations throughout the document? RoboHelp can certainly help there as you would create a variable in one place and then insert it where needed. Updates would then just need one amendment to change it in all locations.
Give us a bit more information and hopefully I or someone else can assist.