@ Jackie_W, BetterRed and itimpi,
THANK YOU! Your suggestion worked. I edited the .rft file in MS Word. I deleted all the tabs in my document and replaced them with a 0.3 indent. I made sure the first paragraphs of each section were a "normal" style so that they would not be indented.
Then I imported the .rtf directly into Calibre and converted it. By default, Calibre still inserted a line space between the paragrah. I re-ran the conversion, this time checking the "do not insert a space" box and setting Calibre's indent to "no change". This time, the document came out looking exactly like the original file.
I truly appreciate all of your gracious, constructive suggestions, and your patience with my limited technical knowledge.