I disagree with Opus. It probably should be tax free.
A "flat monthly rate" is usually just the Federal per-diem amounts (or less). A Per Diem is used in place of receipts, etc. So all you have to 'prove' is that you were out-of-town on business and the satisfies the requirement of the Accountable Plan and would it be tax-free.
As I said before, check with your employer to see if they are doing it under an Accountable Plan. They should be, and pretty much no extra work is required.
