Does the Splunk App for Unix and Linux support Ping to monitor downtime?
I don't think there is a ping sourcetype but there are many other ways of doing that same thing. Do you need to know how long it's been since a host has last communicated with Splunk?
You could write a search for that subtracting the last time any data was captured from current time. But you may find the Monitoring Console's forwarder features more effective for you.