Perhaps this individual thought you meant "Goodyear Endurance RSA" truck tires and didn't know about the Endurance ST tires. If he doesn't know about the latter then it's a good idea to go somewhere else.
NOTE: In the past I have known of several tire shops that will not mount truck tires on trailer wheels, or they will mount them and mark the Work Order ticket "Mounted on Trailer Wheels", which voids any tire warranty.
'07 K3500 Silverado LT Crew Duramax (LBZ)
2016 Salem 27RKSS
1984 CHEV SCOTTSDALE K20 2GCGK24J0E1XXXXXX (Chevrolet Legends-Class of 2019)
"...exhaust fluid? We don't need no stinkin' exhaust fluid"