Vasili, author of several SDR# plugins has recently released a new APCO P25 plugin for SDR#. The plugin is easy to use, simply tune to a P25 voice signal, and it will automatically decode it into voice audio assuming that the signal is not encrypted. If the P25 signal is encrypted, you will hear garbled unintelligible voice. The plugin does not support trunking or any advanced talk group filtering features that you might find with DSD+, Unitrunker, SDR Trunk etc.
To install the plugin, simply download the zip file from rtl-sdr.ru and extra the .dll's into the SDR# folder. Then copy the text in magicline.txt file into the plugins.xml file inside the SDR# folder. The plugin should work with any SDR supported by SDR#, including the RTL-SDR.