The epub standard, as it exists currently, is very specific to fictional stories. Features that are common in other types of books (poetry, textbooks, charts, etc) are simply not adequately supported at this time. Hopefully, as the epub standard matures, these issues will be addressed.
Unfortunately, RPGs are one of the book types that are very seriously handicapped by the current standard. Tables are not available. Images can be used but their zoom rate can be altered - all of which is hardware/software specific. So what is correct for one is wrong for another. Their is almost no universal way to handle things at the moment. Tabs are effectively useless simply because the viewer's screen and font size is going to cause differences in how the text is displayed.
This leaves you with two possibilities. Either maintain a PDF edition, or offer an "Epub & Other" edition. All of the reflowable text is in the epub with references (links perhaps?) to either HTML or PDF pages for the tables. It's neither perfect or pretty, but it should work.