Navistar (International Harvester) is the parent company of Workhorse Custom Chassis. I'm fairly sure an authorized Workhorse Service Center should be able to repair a Maxforce engine even though it is on a Tiffin-built chassis. Workhorse is using the Maxforce 7 on several chassis. Click HERE
to search for a Workhorse Service Center.
You may want to give Tiffin a call and ask them about engine service just to be sure.