A lot of the Workhorse chassis with the 8.1 Chevy had an issue with providing enough airflow past the plug wires. They would get hot/burn and eventually fail. As mentioned, you probably need a tune up and look and see if you can add some ducting or sheet metal to increase airflow.
If you or a mechanic does the work, check the exhaust manifolds while in there. They were known for cracking, also caused by the heat.
Don & Mary
2014 Newmar Dutch Star - All Electric - 450 ISL
2016 GMC Canyon 4WD Crew Cab