How do I bring this up to my manager?

He constantly has me working inside at the register instead of in the garage, working on cars. I know our normal register guy is on vacation but every day, it's me filling for him. I'm no more qualified to run the register than anyone else. He even has me do it when customers are waiting 2 and 3 hours for their car. I can't go out and help to bring the wait time down.

He also has me clean the garage floor at the end of every day even though he once told me, "The person who spills the most oil is the person who cleans the floor". There's oil on the floor right now and it ain't from me because I've been at the register all day. 😑

I feel disrespected.