NPM doesn't currently poll tunnels out of the box unless the device presents them as a logical interface. Do those models present tunnels as logical interfaces?
In most cases, you can create a quick UNDP to get the bandwidth graphed and available for reporting and alerting.
Here's some articles/training to implement cobrien's suggestion. Here is an article that should help you. https://support.solarwinds.com/Success_Center/Network_Performance_Monitor_(NPM)/Add_MIBs_to_the_SolarWinds_MIB_database We also have a training that is relevant. Login to the customer portal, click virtual classrooms, select Orion NPM and find the class named: Expert Series: SNMP, OIDS & UnDP https://customerportal.solarwinds.com/VirtualClassrooms/#