Google "rental property management software"
Most of the major companies have a demo/trial version so you can see if their product fits your needs. Ignore the market-speak on their main web pages. Get as deep into their web site as you can -- a Forum or Tips and Tricks section will be more helpful that the "greatest thing since sliced bread" advertising on the front page.
If your empire is smallish you could probably get by with a well-written Excel template.

