Heavier tires/wheels shouldn’t affect mpg on a long freeway trip. Try that, see if the mpg’s are still reduced. Heavier rotating components (like wheels tires) would show up more if the driving conditions included a lot of accelerating, esp accelerating from stops. Not much accelerating necessary during a long freeway trip at a fairly constant speed.
Also use that freeway trip to check the odometer is accurate. Sometimes messing with the wheels will affect that. Freeways usually have small signs posted along side the roadway exactly every mile. Partly to help the road crew find the exact location where they are supposed to go to work on something. At 60 mph you’ll pass those signs exactly every 60 seconds if the odometer/speedometer is correct. I rented a 2019 Corolla over the summer and I used that technique to notice the odometer was a little off on that rental car. Not much, but definitely not spot on.