It is easier to bend a 2x6 verses 2x12..so wouldn't using 2x12's or 2x10's make it more rigid? maybe I missed something.
I think in Ontario your pressure treated is Pine ? Out here in B.C. pressure treated lumber is mostly Hemlock or fir but mostly Hemlock it becomes water logged during the treating process. When it dries out is when it really splits.I just did a trailer with 2x10 pressure treated.....did the crown up thing, 1/4 in gap between boards. ...Looks good. will see how it lasts.