I'm not very Relm savvy, but I "believe" it takes a certain (now discontinued) version of software to accomplish what you want to do.
Not true. RX-Only (with NO system key required) was added in NeoVision software version 2.1.0 (November 2, 2014) with Firmware Version 5.2.0, and continues to work just fine to this day in current firmware 5.4.0c and Neovision version 2.4.0 for my two KNG-P800 P25 Phase II trunking scanners. I have a total of 16 systems, 33 sites and 787 talkgroups programmed and receiving well on these radios, with no system key access. I don't even have any way to create a system key. I've never even heard of a Relm system key generator being available in the wild (I could be wrong on that).
See
http://www.relmservice.com/Firmware/Files/KNG_Firmware_Notes_5.4.0c.pdf.
Scroll down to 5.2.0 on page 6, where this feature first appeared: "P25 Trunking: RX‐only monitor."
This is an important feature to a number of RELM public safety customers who want to be able to monitor neighboring agencies that do not share system keys. RX-only is a SYSTEM level setting. The software stops asking for a system key once you check the "Rx Only" option below the "Coverage Type" field in the P25 Trunking/General tab for a particular system.
I'm sorry that the OP received bad information from a dealer or a CSR who isn't sufficiently familiar with this feature (but I've offered to provide the e-mail addresses for a sales rep and a software engineer (both with @relm.com email addresses) if anyone would like to discuss with KNOWLEDGEABLE Relm employees before making a purchase of these great radios).
I obviously don't work for Relm, but acquiring these two KNG-P800s (expensive as they were) was by far the single best decision I've made in the course of my 25+ year scanner listening "career," so I confess that I'm a bit of an "evangelist" on this point now.
Of course, I like my Unication G5 too. Just not nearly as much as my KNG-P800 radios.
Dan