If you're interested, I did a write-up and released code for various pfSense non-SMTP checks with Nagios -- CPU, memory, pfSense services, VPN/IPSEC tunnels, interfaces, state table, firmware version, CPU temperature, system uptime, and more. The checks work on both Nagios Core and Nagios XI.
https://www.linuxincluded.com/monitoring-pfsense-with-nagios-xi-using-ssh-part-1/