I'll be in the Avalon by Friday morning for vacation and doing support of Targa operations. I believe that the PSR800 may need you to change a couple of settings in the default configuration in order to monitor the P25 trunk properly. There's a band plan setting accessed via EZ-Scan software, in the site options for each site (if I recall correctly).. make sure it is set to "Auto" instead of "Standard" (or whatever it defaults to). NPR P25 does not use a standard band plan, and I'm pretty sure you will not get any comms if you have it set to Standard.
I have successfully monitored the P25 system with a PSR800, a PSR500 (trunk CC dump into Unitrunker only), and a 396XT and a HomePatrol. After Targa is over, I'll be around for a couple weeks, and I'll be trying to flesh out more of the sites and pin them down in the right places.