This was the OID I was looking for. Thanks!
I still had to figure out how to query the route table pertaining to a specific routing-instance and the solution is to reference the routing-instance as part of the community string as follows:
snmpwalk -v <version> -c <routing-instance>@<community> <hostname/IP> <OID>
The key is to put the routing-instance name before the community string separated by the "@" sign.
Example: snmpwalk -v 2c -c iptv@public 10.100.100.1 1.3.6.1.3.60.1.1.2
I also found OIDs relating to IGMP and PIM.
IGM: https://apps.juniper.net/mib-explorer/navigate.jsp#object=igmpStdMIB&product=Junos%20OS&release=12.3R12
PIM: https://apps.juniper.net/mib-explorer/navigate.jsp#object=pimMIB&product=Junos%20OS&release=12.3R12