Construction workers living in their RV on the jobsite. Do they have to take the actual expenses or can they take the daily GSA perdiem? Of course they are better off with the daily per diem but not sure if that is possible. (Legal) They do have a tax home in another state.

No.

The lodging per diem is NOT like the food and MISC per diem. It is NOT intended to be used in lieu of actual hotel receipt, but is a comparison tool for the IRS.

The actual expenses for the RV must be used.