If anyone is using iSCSI with FreeNAS and are having issues with their drives randomly disconnecting or getting "no ping reply (NOP-Out) after 5 seconds" messages try adding the following under system then tunables...

Variable = kern.cam.ctl.iscsi.ping_timeout

Value = 0

Type = sysctl

Haven't been able to track down a root cause but it's been 8 days and still no issues.