I really want to just go put something under the wheels but I’ve never talked to these people.
Update edit: thank you for all the replies. I went over and knocked but no answer. It’s definitely not chocked. If I see them come home I’ll go back over. I’m worried as soon as the weight shifts loading it that it will just go down the hill.
Go for it, use a personal anecdote if you have one about how you or someone you know was bit by not putting blocks in front of the wheels.
If you don’t have one… make up a white lie about how your now deceased great uncle Johnny told you he left a trailer out without blocks on a little slope like that and it rolled into the street.
The anecdotes / a story can be a … easier path to selling the truth that can feel a bit more humbled vs “I know better than you, so do what I say.”