Ian, the fan is obviously not the problem since it does work just not when
you want it to.  I have found that you can almost always trace this problem
to the A/C relay found inside your rear cabinet in the camper.  PN is
171959141A.  If you have a known good one to swap in this is an excellent way
to check.  I sell these relays for $12 each.  Another sign that this relay is
bad is that your 3rd speed on your evaporator fan might not work.
Hope this helps you.
When the A/C compressor comes on the fan in the front should automatically
come on no matter what.
Ken Wilford
Van-Again
John 3:16