External monitor issues

Silly question, but will this script work if you run it directly from the command line, with an IP address parameter? You're using an alias service port in your monitor so you shouldn't have an issue applying the monitor to your IP:0 pool. Otherwise, I've tested it on 11.3 and don't see anything specifically wrong with your config. I did make a minor modification though to react to a specific remote response vs. an exit status.

!/bin/sh
 remove IPv6/IPv4 compatibility prefix (LTM passes addresses in IPv6 format)
IP=`echo ${1} | sed 's/::ffff://'`

 set the pid file location and name
PIDFILE="/var/run/`basename ${0}`.${IP}.${PORT}.pid"

if [ -f $PIDFILE ]
then
     kill -9 -`cat $PIDFILE` > /dev/null 2>&1
fi
echo "$$" > $PIDFILE

 send request & check for expected response
SSHRES=`ssh splunkf5@${IP} -i /config/ssh/ssh_host_dsa_key /home/splunkf5/healthcheck.sh`

 mark node UP if expected response was received
if [ $SSHRES == "good" ]
then
     echo "UP"
fi

rm -f $PIDFILE
exit