It's an article of faith among Florida gun rights activists that this indeed did happen. Muggers and carjackers circa late 80s/early 90s were also targeting rental cars, often coming out of airports; at the time in Florida, rental cars had distinctive license plates identifying them as such. It was believed that the bad guys had read in the papers and seen on TV news stories of their kind being blown away by armed citizens, who had gotten shall-issue permits in the late 1980s thanks to the hard work of Marion Hammer and her group, Unified Sportsmen of Florida (USFinfo@aol.com). At the time, permits were for residents only, which made tourists all the more attractive as targets.
This particular crime phenomenon faded out when (a)the rental cars were given more conventional looking plates, (b)Florida Highway Patrol began stationing uniformed troopers at rest stops, and (c)Florida advanced its gun laws to include both non-resident permits and limited reciprocity.
I just got off the phone with Marion Hammer at Unified Sportsmen of Florida. She confirms that it was all over the newspapers at the time (again, late 1980s-early 1990s), and suggests an archive search of major Florida newspapers during that period. She tells me it's all anecdotal material, and does not recall any major paper or study being published on the phenomenon.
Hoping that helps answer your question,