I have a missfire on #4 cylider but truck is running fine slight miss when idiling has 220,000 miles pulling a small pop up camper regular maintance never had problem before its an 07 with 5.7 hemi

Missfire on #4 cylinder truck runs fine no loss of power barley noticeable rough idial
when parked truck has 220,000hiway miles pulling small pop up well kept up and
maintained never had a check engine light before has had regular plug changes but
never coil packs or boots truck has had a slight tic from 40,000 miles dealer could find
no problems said it was the "hemi tic" i run synthetic oil and high octane fuel where
should i start looking for the problem
Thanks Elmo