How long will a treated post last in the ground?
A PT post will last a long time in concrete, maybe 5 to 10 years in soil alone.  I suggest you embed the post in concrete, trowel a peak around the post so water runs off, and don't let the PT post come in contact with the ground.

If you find the RV posts I mentioned earlier, they are steel, treated and when put in concrete will last 25 years or more.