@chrcoluk
I have discovered a work-around that seems to work. AirVPN assigns my pfSense firewall an IP address in the 10.0.0.0/8 CIDR based on the server pfSense is connected to. For example, I may get an address like 10.52.68.42. If I change the last digit to 1 (i.e., 10.52.68.1), and insert the result IP address into the Monitor IP field of the gateway settings, I get proper ping times. I believe the X.X.X.1 effectively specifies the internal address of AirVPN's respective gateway.
Unfortunately, this work-around is not a complete solution to my problem. In my OpenVPN configuration, I actually have four AirVPN server connections active. A first pair corresponds to one physical location (e.g., New York, NY) and a second pair corresponds to another physical location (e.g., Newark, NJ). I choose the physical locations based on their corresponding servers ping times, namely, the first pair has the lowest ping times and the second pair has the next lowest ping times. pfSense is configured to load balance the servers within each pair, and the higher latency pair serves as a failover to the lower latency pair.
If try to set the Monitor IP of each respective gateway to X.X.X.1, I get proper latency values for only one (and sometimes two) AirVPN servers. The others are listed as offline. So the work-around seems to function okay for one active server, but with more than one, pfSense seems to have issues.