From hearing these stories, I have found that doormats, chocks, and such are not sufficient to "mark your space". Whether it's because they are flat and not really visible or what, they just don't say "occupied".
Trash bag and firewood - I would not really consider them as indicators of occupation at any time.
Seems chairs, tents, etc., set up for use (not just laying on the parking pad) are better indicators.
I have not had this problem (I don't usually drive away from my site), but if I did, I would make a simple "Occupied" sign and laminate it. Hang that on the post with the small envelope tag showing the dates on top of it. That will have "occupied" very visible and the official tag with dates available for closer inspection. Normally I reserve online for popular campgrounds so my site usually has the big tag on it that makes it easy to know it's taken.