I use SDR#, with a Virtual Cable (downloadable software that works in the background to pipe the digital audio from SDR# to DSD+), connected to DSD+ Fastlane for finding and decoding DMR, NXDN, and P25 systems.
For P25 sites I use the Unitrunker software for logging Radio IDs and Talkgroups. You can use Unitrunker for all your P25 needs, but I just find it easier to search and find P25 systems and sites on SDR#/DSD+, and then switch to Unitrunker for gathering and logging RIDs and TGs. IMO, Unitrunker is visually more appealing with its customizable colored tags for easy deciphering of individual Radio IDs and Talkgroups for P25 sites.
Those are the 4 pieces of software that I have used with my RTL-SDR. There are other softwares, such as OP25, and SDRTrunk, but I've never used those, and don't even know what their user-interface looks like.
As far as not having any digital radio systems locally, I bet you have some; most states have a statewide P25 radio system. Whether or not your local agencies use it may be a different story, but you'll still be able to gather data from the sites and systems. A lot of states and regions have similar wide-area DMR or NXDN radio systems that businesses can lease channels on to use for their business. Typical users of those business systems would be businesses that use many vehicles, such as school buses, garbage trucks, taxis, tow truck companies, public transit buses, private ambulance transportation companies, road construction crews, etc.. Here in my area, all of the large hospital networks (each with hospitals in several different counties) have their own digital radio system that links all of their hospitals together. Also if you're near any military installations or General Motors automotive plants, they have large, nationwide digital radio systems; as I'm sure many other large, nationwide businesses do as well.
With an outdoor antenna connected to your SDR dongle you'll be amazed at how many digital radio systems you'll find. I have an outdoor antenna at 25ft, on top of my house in east-central Indiana, and I've found and decoded radio systems from Ontario Canada, to Wisconsin, to Missouri, to Virginia, and everywhere in between, along with dozens of sites and systems around my local counties.
I also take my laptop and SDR in my vehicle to chase and track down new sites and systems to help build-out the RRDB. Duke Energy is building a new nationwide P25 system in several states, several of the sites were online in southern Indiana but they were not listed on RR, so my other-half and I took a roadtrip. When we got into the target-area, as she drove down the highway, I found and logged the new Duke sites on the laptop, then submitted my finding to RR that evening.
Needless to say, I enjoy hunting for new systems and sites 🤣, and RR could always use more people submitting accurate data.
